graphix_compiler/typ/

mod.rs

use crate::{
    env::Env, expr::ModPath, format_with_flags, PrintFlag, Rt, UserEvent, PRINT_FLAGS,
};
use anyhow::{anyhow, bail, Result};
use arcstr::ArcStr;
use enumflags2::bitflags;
use enumflags2::BitFlags;
use fxhash::{FxHashMap, FxHashSet};
use immutable_chunkmap::map::Map;
use netidx::{
    publisher::{Typ, Value},
    utils::Either,
};
use netidx_value::ValArray;
use poolshark::local::LPooled;
use smallvec::{smallvec, SmallVec};
use std::{
    cmp::{Eq, PartialEq},
    collections::hash_map::Entry,
    fmt::{self, Debug},
    iter,
};
use triomphe::Arc;

mod fntyp;
mod tval;
mod tvar;

pub use fntyp::{FnArgType, FnType};
pub use tval::TVal;
use tvar::would_cycle_inner;
pub use tvar::TVar;

#[derive(Debug, Clone, Copy)]
#[bitflags]
#[repr(u8)]
pub enum ContainsFlags {
    AliasTVars,
    InitTVars,
}

struct AndAc(bool);

impl FromIterator<bool> for AndAc {
    fn from_iter<T: IntoIterator<Item = bool>>(iter: T) -> Self {
        AndAc(iter.into_iter().all(|b| b))
    }
}

#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord)]
pub enum Type {
    Bottom,
    Any,
    Primitive(BitFlags<Typ>),
    Ref { scope: ModPath, name: ModPath, params: Arc<[Type]> },
    Fn(Arc<FnType>),
    Set(Arc<[Type]>),
    TVar(TVar),
    Error(Arc<Type>),
    Array(Arc<Type>),
    ByRef(Arc<Type>),
    Tuple(Arc<[Type]>),
    Struct(Arc<[(ArcStr, Type)]>),
    Variant(ArcStr, Arc<[Type]>),
    Map { key: Arc<Type>, value: Arc<Type> },
}

impl Default for Type {
    fn default() -> Self {
        Self::Bottom
    }
}

impl Type {
    pub fn empty_tvar() -> Self {
        Type::TVar(TVar::default())
    }

    fn iter_prims(&self) -> impl Iterator<Item = Self> {
        match self {
            Self::Primitive(p) => {
                Either::Left(p.iter().map(|t| Type::Primitive(t.into())))
            }
            t => Either::Right(iter::once(t.clone())),
        }
    }

    pub fn is_defined(&self) -> bool {
        match self {
            Self::Bottom
            | Self::Any
            | Self::Primitive(_)
            | Self::Fn(_)
            | Self::Set(_)
            | Self::Error(_)
            | Self::Array(_)
            | Self::ByRef(_)
            | Self::Tuple(_)
            | Self::Struct(_)
            | Self::Variant(_, _)
            | Self::Ref { .. }
            | Self::Map { .. } => true,
            Self::TVar(tv) => tv.read().typ.read().is_some(),
        }
    }

    pub fn lookup_ref<'a, R: Rt, E: UserEvent>(
        &'a self,
        env: &'a Env<R, E>,
    ) -> Result<&'a Type> {
        match self {
            Self::Ref { scope, name, params } => {
                let def = env
                    .lookup_typedef(scope, name)
                    .ok_or_else(|| anyhow!("undefined type {name} in {scope}"))?;
                if def.params.len() != params.len() {
                    bail!("{} expects {} type parameters", name, def.params.len());
                }
                def.typ.unbind_tvars();
                for ((tv, ct), arg) in def.params.iter().zip(params.iter()) {
                    if let Some(ct) = ct {
                        ct.check_contains(env, arg)?;
                    }
                    if !tv.would_cycle(arg) {
                        *tv.read().typ.write() = Some(arg.clone());
                    }
                }
                Ok(&def.typ)
            }
            t => Ok(t),
        }
    }

    pub fn check_contains<R: Rt, E: UserEvent>(
        &self,
        env: &Env<R, E>,
        t: &Self,
    ) -> Result<()> {
        if self.contains(env, t)? {
            Ok(())
        } else {
            format_with_flags(PrintFlag::DerefTVars | PrintFlag::ReplacePrims, || {
                bail!("type mismatch {self} does not contain {t}")
            })
        }
    }

    fn contains_int<R: Rt, E: UserEvent>(
        &self,
        flags: BitFlags<ContainsFlags>,
        env: &Env<R, E>,
        hist: &mut FxHashMap<(usize, usize), bool>,
        t: &Self,
    ) -> Result<bool> {
        if (self as *const Type) == (t as *const Type) {
            return Ok(true);
        }
        match (self, t) {
            (
                Self::Ref { scope: s0, name: n0, params: p0 },
                Self::Ref { scope: s1, name: n1, params: p1 },
            ) if s0 == s1 && n0 == n1 => Ok(p0.len() == p1.len()
                && p0
                    .iter()
                    .zip(p1.iter())
                    .map(|(t0, t1)| t0.contains_int(flags, env, hist, t1))
                    .collect::<Result<AndAc>>()?
                    .0),
            (t0 @ Self::Ref { .. }, t1) | (t0, t1 @ Self::Ref { .. }) => {
                let t0 = t0.lookup_ref(env)?;
                let t1 = t1.lookup_ref(env)?;
                let t0_addr = (t0 as *const Type).addr();
                let t1_addr = (t1 as *const Type).addr();
                match hist.get(&(t0_addr, t1_addr)) {
                    Some(r) => Ok(*r),
                    None => {
                        hist.insert((t0_addr, t1_addr), true);
                        match t0.contains_int(flags, env, hist, t1) {
                            Ok(r) => {
                                hist.insert((t0_addr, t1_addr), r);
                                Ok(r)
                            }
                            Err(e) => {
                                hist.remove(&(t0_addr, t1_addr));
                                Err(e)
                            }
                        }
                    }
                }
            }
            (Self::TVar(t0), Self::Bottom) => {
                if let Some(_) = &*t0.read().typ.read() {
                    return Ok(true);
                }
                if flags.contains(ContainsFlags::InitTVars) {
                    *t0.read().typ.write() = Some(Self::Bottom);
                }
                Ok(true)
            }
            (Self::TVar(t0), Self::Any) => {
                if let Some(t0) = &*t0.read().typ.read() {
                    return t0.contains_int(flags, env, hist, t);
                }
                if flags.contains(ContainsFlags::InitTVars) {
                    *t0.read().typ.write() = Some(Self::Any);
                }
                Ok(true)
            }
            (Self::Any, _) => Ok(true),
            (Self::Bottom, _) | (_, Self::Bottom) => Ok(true),
            (Self::Primitive(p0), Self::Primitive(p1)) => Ok(p0.contains(*p1)),
            (
                Self::Primitive(p),
                Self::Array(_) | Self::Tuple(_) | Self::Struct(_) | Self::Variant(_, _),
            ) => Ok(p.contains(Typ::Array)),
            (Self::Array(t0), Self::Array(t1)) => t0.contains_int(flags, env, hist, t1),
            (Self::Array(t0), Self::Primitive(p)) if *p == BitFlags::from(Typ::Array) => {
                t0.contains_int(flags, env, hist, &Type::Any)
            }
            (Self::Map { key: k0, value: v0 }, Self::Map { key: k1, value: v1 }) => {
                Ok(k0.contains_int(flags, env, hist, k1)?
                    && v0.contains_int(flags, env, hist, v1)?)
            }
            (Self::Primitive(p), Self::Map { .. }) => Ok(p.contains(Typ::Map)),
            (Self::Map { key, value }, Self::Primitive(p))
                if *p == BitFlags::from(Typ::Map) =>
            {
                Ok(key.contains_int(flags, env, hist, &Type::Any)?
                    && value.contains_int(flags, env, hist, &Type::Any)?)
            }
            (Self::Primitive(p0), Self::Error(_)) => Ok(p0.contains(Typ::Error)),
            (Self::Error(e), Self::Primitive(p)) if *p == BitFlags::from(Typ::Error) => {
                e.contains_int(flags, env, hist, &Type::Any)
            }
            (Self::Error(e0), Self::Error(e1)) => e0.contains_int(flags, env, hist, e1),
            (Self::Tuple(t0), Self::Tuple(t1))
                if t0.as_ptr().addr() == t1.as_ptr().addr() =>
            {
                Ok(true)
            }
            (Self::Tuple(t0), Self::Tuple(t1)) => Ok(t0.len() == t1.len()
                && t0
                    .iter()
                    .zip(t1.iter())
                    .map(|(t0, t1)| t0.contains_int(flags, env, hist, t1))
                    .collect::<Result<AndAc>>()?
                    .0),
            (Self::Struct(t0), Self::Struct(t1))
                if t0.as_ptr().addr() == t1.as_ptr().addr() =>
            {
                Ok(true)
            }
            (Self::Struct(t0), Self::Struct(t1)) => {
                Ok(t0.len() == t1.len() && {
                    // struct types are always sorted by field name
                    t0.iter()
                        .zip(t1.iter())
                        .map(|((n0, t0), (n1, t1))| {
                            Ok(n0 == n1 && t0.contains_int(flags, env, hist, t1)?)
                        })
                        .collect::<Result<AndAc>>()?
                        .0
                })
            }
            (Self::Variant(tg0, t0), Self::Variant(tg1, t1))
                if tg0.as_ptr() == tg1.as_ptr()
                    && t0.as_ptr().addr() == t1.as_ptr().addr() =>
            {
                Ok(true)
            }
            (Self::Variant(tg0, t0), Self::Variant(tg1, t1)) => Ok(tg0 == tg1
                && t0.len() == t1.len()
                && t0
                    .iter()
                    .zip(t1.iter())
                    .map(|(t0, t1)| t0.contains_int(flags, env, hist, t1))
                    .collect::<Result<AndAc>>()?
                    .0),
            (Self::ByRef(t0), Self::ByRef(t1)) => t0.contains_int(flags, env, hist, t1),
            (Self::TVar(t0), Self::TVar(t1))
                if t0.addr() == t1.addr() || t0.read().id == t1.read().id =>
            {
                Ok(true)
            }
            (Self::TVar(t0), tt1 @ Self::TVar(t1)) => {
                #[derive(Debug)]
                enum Act {
                    RightCopy,
                    RightAlias,
                    LeftAlias,
                    LeftCopy,
                }
                let act = {
                    let t0 = t0.read();
                    let t1 = t1.read();
                    let addr = Arc::as_ptr(&t0.typ).addr();
                    if addr == Arc::as_ptr(&t1.typ).addr() {
                        return Ok(true);
                    }
                    let t0i = t0.typ.read();
                    let t1i = t1.typ.read();
                    match (&*t0i, &*t1i) {
                        (Some(t0), Some(t1)) => {
                            return t0.contains_int(flags, env, hist, &*t1)
                        }
                        (None, None) => {
                            if would_cycle_inner(addr, tt1) {
                                return Ok(true);
                            }
                            if t0.frozen && t1.frozen {
                                return Ok(true);
                            }
                            if t0.frozen {
                                Act::RightAlias
                            } else {
                                Act::LeftAlias
                            }
                        }
                        (Some(_), None) => {
                            if would_cycle_inner(addr, tt1) {
                                return Ok(true);
                            }
                            Act::RightCopy
                        }
                        (None, Some(_)) => {
                            if would_cycle_inner(addr, tt1) {
                                return Ok(true);
                            }
                            Act::LeftCopy
                        }
                    }
                };
                match act {
                    Act::RightCopy if flags.contains(ContainsFlags::InitTVars) => {
                        t1.copy(t0)
                    }
                    Act::RightAlias if flags.contains(ContainsFlags::AliasTVars) => {
                        t1.alias(t0)
                    }
                    Act::LeftAlias if flags.contains(ContainsFlags::AliasTVars) => {
                        t0.alias(t1)
                    }
                    Act::LeftCopy if flags.contains(ContainsFlags::InitTVars) => {
                        t0.copy(t1)
                    }
                    Act::RightCopy | Act::RightAlias | Act::LeftAlias | Act::LeftCopy => {
                        ()
                    }
                }
                Ok(true)
            }
            (Self::TVar(t0), t1) if !t0.would_cycle(t1) => {
                if let Some(t0) = &*t0.read().typ.read() {
                    return t0.contains_int(flags, env, hist, t1);
                }
                if flags.contains(ContainsFlags::InitTVars) {
                    *t0.read().typ.write() = Some(t1.clone());
                }
                Ok(true)
            }
            (t0, Self::TVar(t1)) if !t1.would_cycle(t0) => {
                if let Some(t1) = &*t1.read().typ.read() {
                    return t0.contains_int(flags, env, hist, t1);
                }
                if flags.contains(ContainsFlags::InitTVars) {
                    *t1.read().typ.write() = Some(t0.clone());
                }
                Ok(true)
            }
            (Self::Set(s0), Self::Set(s1))
                if s0.as_ptr().addr() == s1.as_ptr().addr() =>
            {
                Ok(true)
            }
            (t0, Self::Set(s)) => Ok(s
                .iter()
                .map(|t1| t0.contains_int(flags, env, hist, t1))
                .collect::<Result<AndAc>>()?
                .0),
            (Self::Set(s), t) => Ok(s
                .iter()
                .fold(Ok::<_, anyhow::Error>(false), |acc, t0| {
                    Ok(acc? || t0.contains_int(flags, env, hist, t)?)
                })?
                || t.iter_prims().fold(Ok::<_, anyhow::Error>(true), |acc, t1| {
                    Ok(acc?
                        && s.iter().fold(Ok::<_, anyhow::Error>(false), |acc, t0| {
                            Ok(acc? || t0.contains_int(flags, env, hist, &t1)?)
                        })?)
                })?),
            (Self::Fn(f0), Self::Fn(f1)) => {
                Ok(f0.as_ptr() == f1.as_ptr() || f0.contains_int(flags, env, hist, f1)?)
            }
            (_, Self::Any)
            | (_, Self::TVar(_))
            | (Self::TVar(_), _)
            | (Self::Fn(_), _)
            | (Self::ByRef(_), _)
            | (_, Self::ByRef(_))
            | (_, Self::Fn(_))
            | (Self::Tuple(_), Self::Array(_))
            | (Self::Tuple(_), Self::Primitive(_))
            | (Self::Tuple(_), Self::Struct(_))
            | (Self::Tuple(_), Self::Variant(_, _))
            | (Self::Tuple(_), Self::Error(_))
            | (Self::Tuple(_), Self::Map { .. })
            | (Self::Array(_), Self::Primitive(_))
            | (Self::Array(_), Self::Tuple(_))
            | (Self::Array(_), Self::Struct(_))
            | (Self::Array(_), Self::Variant(_, _))
            | (Self::Array(_), Self::Error(_))
            | (Self::Array(_), Self::Map { .. })
            | (Self::Struct(_), Self::Array(_))
            | (Self::Struct(_), Self::Primitive(_))
            | (Self::Struct(_), Self::Tuple(_))
            | (Self::Struct(_), Self::Variant(_, _))
            | (Self::Struct(_), Self::Error(_))
            | (Self::Struct(_), Self::Map { .. })
            | (Self::Variant(_, _), Self::Array(_))
            | (Self::Variant(_, _), Self::Struct(_))
            | (Self::Variant(_, _), Self::Primitive(_))
            | (Self::Variant(_, _), Self::Tuple(_))
            | (Self::Variant(_, _), Self::Error(_))
            | (Self::Variant(_, _), Self::Map { .. })
            | (Self::Error(_), Self::Array(_))
            | (Self::Error(_), Self::Primitive(_))
            | (Self::Error(_), Self::Struct(_))
            | (Self::Error(_), Self::Variant(_, _))
            | (Self::Error(_), Self::Tuple(_))
            | (Self::Error(_), Self::Map { .. })
            | (Self::Map { .. }, Self::Array(_))
            | (Self::Map { .. }, Self::Primitive(_))
            | (Self::Map { .. }, Self::Struct(_))
            | (Self::Map { .. }, Self::Variant(_, _))
            | (Self::Map { .. }, Self::Tuple(_))
            | (Self::Map { .. }, Self::Error(_)) => Ok(false),
        }
    }

    pub fn contains<R: Rt, E: UserEvent>(
        &self,
        env: &Env<R, E>,
        t: &Self,
    ) -> Result<bool> {
        self.contains_int(
            ContainsFlags::AliasTVars | ContainsFlags::InitTVars,
            env,
            &mut LPooled::take(),
            t,
        )
    }

    pub fn contains_with_flags<R: Rt, E: UserEvent>(
        &self,
        flags: BitFlags<ContainsFlags>,
        env: &Env<R, E>,
        t: &Self,
    ) -> Result<bool> {
        self.contains_int(flags, env, &mut LPooled::take(), t)
    }

    fn could_match_int<R: Rt, E: UserEvent>(
        &self,
        env: &Env<R, E>,
        hist: &mut FxHashMap<(usize, usize), bool>,
        t: &Self,
    ) -> Result<bool> {
        let fl = BitFlags::empty();
        match (self, t) {
            (
                Self::Ref { scope: s0, name: n0, params: p0 },
                Self::Ref { scope: s1, name: n1, params: p1 },
            ) if s0 == s1 && n0 == n1 => Ok(p0.len() == p1.len()
                && p0
                    .iter()
                    .zip(p1.iter())
                    .map(|(t0, t1)| t0.could_match_int(env, hist, t1))
                    .collect::<Result<AndAc>>()?
                    .0),
            (t0 @ Self::Ref { .. }, t1) | (t0, t1 @ Self::Ref { .. }) => {
                let t0 = t0.lookup_ref(env)?;
                let t1 = t1.lookup_ref(env)?;
                let t0_addr = (t0 as *const Type).addr();
                let t1_addr = (t1 as *const Type).addr();
                match hist.get(&(t0_addr, t1_addr)) {
                    Some(r) => Ok(*r),
                    None => {
                        hist.insert((t0_addr, t1_addr), true);
                        match t0.could_match_int(env, hist, t1) {
                            Ok(r) => {
                                hist.insert((t0_addr, t1_addr), r);
                                Ok(r)
                            }
                            Err(e) => {
                                hist.remove(&(t0_addr, t1_addr));
                                Err(e)
                            }
                        }
                    }
                }
            }
            (t0, Self::Primitive(s)) => {
                for t1 in s.iter() {
                    if t0.contains_int(fl, env, hist, &Type::Primitive(t1.into()))? {
                        return Ok(true);
                    }
                }
                Ok(false)
            }
            (Type::Primitive(p), Type::Error(_)) => Ok(p.contains(Typ::Error)),
            (Type::Error(t0), Type::Error(t1)) => t0.could_match_int(env, hist, t1),
            (Type::Array(t0), Type::Array(t1)) => t0.could_match_int(env, hist, t1),
            (Type::Primitive(p), Type::Array(_)) => Ok(p.contains(Typ::Array)),
            (Type::Map { key: k0, value: v0 }, Type::Map { key: k1, value: v1 }) => {
                Ok(k0.could_match_int(env, hist, k1)?
                    && v0.could_match_int(env, hist, v1)?)
            }
            (Type::Primitive(p), Type::Map { .. }) => Ok(p.contains(Typ::Map)),
            (Type::Tuple(ts0), Type::Tuple(ts1)) => Ok(ts0.len() == ts1.len()
                && ts0
                    .iter()
                    .zip(ts1.iter())
                    .map(|(t0, t1)| t0.could_match_int(env, hist, t1))
                    .collect::<Result<AndAc>>()?
                    .0),
            (Type::Struct(ts0), Type::Struct(ts1)) => Ok(ts0.len() == ts1.len()
                && ts0
                    .iter()
                    .zip(ts1.iter())
                    .map(|((n0, t0), (n1, t1))| {
                        Ok(n0 == n1 && t0.could_match_int(env, hist, t1)?)
                    })
                    .collect::<Result<AndAc>>()?
                    .0),
            (Type::Variant(n0, ts0), Type::Variant(n1, ts1)) => Ok(ts0.len()
                == ts1.len()
                && n0 == n1
                && ts0
                    .iter()
                    .zip(ts1.iter())
                    .map(|(t0, t1)| t0.could_match_int(env, hist, t1))
                    .collect::<Result<AndAc>>()?
                    .0),
            (Type::ByRef(t0), Type::ByRef(t1)) => t0.could_match_int(env, hist, t1),
            (t0, Self::Set(ts)) => {
                for t1 in ts.iter() {
                    if t0.could_match_int(env, hist, t1)? {
                        return Ok(true);
                    }
                }
                Ok(false)
            }
            (Type::Set(ts), t1) => {
                for t0 in ts.iter() {
                    if t0.could_match_int(env, hist, t1)? {
                        return Ok(true);
                    }
                }
                Ok(false)
            }
            (Type::TVar(t0), t1) => match &*t0.read().typ.read() {
                Some(t0) => t0.could_match_int(env, hist, t1),
                None => Ok(true),
            },
            (t0, Type::TVar(t1)) => match &*t1.read().typ.read() {
                Some(t1) => t0.could_match_int(env, hist, t1),
                None => Ok(true),
            },
            (Type::Any, _) | (_, Type::Any) | (Type::Bottom, _) | (_, Type::Bottom) => {
                Ok(true)
            }
            (Type::Fn(_), _)
            | (_, Type::Fn(_))
            | (Type::Tuple(_), _)
            | (_, Type::Tuple(_))
            | (Type::Struct(_), _)
            | (_, Type::Struct(_))
            | (Type::Variant(_, _), _)
            | (_, Type::Variant(_, _))
            | (Type::ByRef(_), _)
            | (_, Type::ByRef(_))
            | (Type::Array(_), _)
            | (_, Type::Array(_))
            | (_, Type::Map { .. })
            | (Type::Map { .. }, _) => Ok(false),
        }
    }

    pub fn could_match<R: Rt, E: UserEvent>(
        &self,
        env: &Env<R, E>,
        t: &Self,
    ) -> Result<bool> {
        self.could_match_int(env, &mut LPooled::take(), t)
    }

    fn union_int<R: Rt, E: UserEvent>(
        &self,
        env: &Env<R, E>,
        hist: &mut FxHashMap<(usize, usize), Type>,
        t: &Self,
    ) -> Result<Self> {
        match (self, t) {
            (
                Type::Ref { scope: s0, name: n0, params: p0 },
                Type::Ref { scope: s1, name: n1, params: p1 },
            ) if n0 == n1 && s0 == s1 && p0.len() == p1.len() => {
                let mut params = p0
                    .iter()
                    .zip(p1.iter())
                    .map(|(p0, p1)| p0.union_int(env, hist, p1))
                    .collect::<Result<LPooled<Vec<_>>>>()?;
                let params = Arc::from_iter(params.drain(..));
                Ok(Self::Ref { scope: s0.clone(), name: n0.clone(), params })
            }
            (tr @ Type::Ref { .. }, t) => {
                let t0 = tr.lookup_ref(env)?;
                let t0_addr = (t0 as *const Type).addr();
                let t_addr = (t as *const Type).addr();
                match hist.get(&(t0_addr, t_addr)) {
                    Some(t) => Ok(t.clone()),
                    None => {
                        hist.insert((t0_addr, t_addr), tr.clone());
                        let r = t0.union_int(env, hist, t)?;
                        hist.insert((t0_addr, t_addr), r.clone());
                        Ok(r)
                    }
                }
            }
            (t, tr @ Type::Ref { .. }) => {
                let t1 = tr.lookup_ref(env)?;
                let t1_addr = (t1 as *const Type).addr();
                let t_addr = (t as *const Type).addr();
                match hist.get(&(t_addr, t1_addr)) {
                    Some(t) => Ok(t.clone()),
                    None => {
                        hist.insert((t_addr, t1_addr), tr.clone());
                        let r = t.union_int(env, hist, t1)?;
                        hist.insert((t_addr, t1_addr), r.clone());
                        Ok(r)
                    }
                }
            }
            (Type::Bottom, t) | (t, Type::Bottom) => Ok(t.clone()),
            (Type::Any, _) | (_, Type::Any) => Ok(Type::Any),
            (Type::Primitive(p), t) | (t, Type::Primitive(p)) if p.is_empty() => {
                Ok(t.clone())
            }
            (Type::Primitive(s0), Type::Primitive(s1)) => {
                let mut s = *s0;
                s.insert(*s1);
                Ok(Type::Primitive(s))
            }
            (
                Type::Primitive(p),
                Type::Array(_) | Type::Struct(_) | Type::Tuple(_) | Type::Variant(_, _),
            )
            | (
                Type::Array(_) | Type::Struct(_) | Type::Tuple(_) | Type::Variant(_, _),
                Type::Primitive(p),
            ) if p.contains(Typ::Array) => Ok(Type::Primitive(*p)),
            (Type::Primitive(p), Type::Array(t))
            | (Type::Array(t), Type::Primitive(p)) => Ok(Type::Set(Arc::from_iter([
                Type::Primitive(*p),
                Type::Array(t.clone()),
            ]))),
            (t @ Type::Array(t0), u @ Type::Array(t1)) => {
                if t0 == t1 {
                    Ok(Type::Array(t0.clone()))
                } else {
                    Ok(Type::Set(Arc::from_iter([t.clone(), u.clone()])))
                }
            }
            (Type::Primitive(p), Type::Map { .. })
            | (Type::Map { .. }, Type::Primitive(p))
                if p.contains(Typ::Map) =>
            {
                Ok(Type::Primitive(*p))
            }
            (Type::Primitive(p), Type::Map { key, value })
            | (Type::Map { key, value }, Type::Primitive(p)) => {
                Ok(Type::Set(Arc::from_iter([
                    Type::Primitive(*p),
                    Type::Map { key: key.clone(), value: value.clone() },
                ])))
            }
            (
                t @ Type::Map { key: k0, value: v0 },
                u @ Type::Map { key: k1, value: v1 },
            ) => {
                if k0 == k1 && v0 == v1 {
                    Ok(Type::Map { key: k0.clone(), value: v0.clone() })
                } else {
                    Ok(Type::Set(Arc::from_iter([t.clone(), u.clone()])))
                }
            }
            (t @ Type::Map { .. }, u) | (u, t @ Type::Map { .. }) => {
                Ok(Type::Set(Arc::from_iter([t.clone(), u.clone()])))
            }
            (Type::Primitive(p), Type::Error(_))
            | (Type::Error(_), Type::Primitive(p))
                if p.contains(Typ::Error) =>
            {
                Ok(Type::Primitive(*p))
            }
            (Type::Error(e0), Type::Error(e1)) => {
                Ok(Type::Error(Arc::new(e0.union_int(env, hist, e1)?)))
            }
            (e @ Type::Error(_), t) | (t, e @ Type::Error(_)) => {
                Ok(Type::Set(Arc::from_iter([e.clone(), t.clone()])))
            }
            (t @ Type::ByRef(t0), u @ Type::ByRef(t1)) => {
                if t0 == t1 {
                    Ok(Type::ByRef(t0.clone()))
                } else {
                    Ok(Type::Set(Arc::from_iter([u.clone(), t.clone()])))
                }
            }
            (Type::Set(s0), Type::Set(s1)) => Ok(Type::Set(Arc::from_iter(
                s0.iter().cloned().chain(s1.iter().cloned()),
            ))),
            (Type::Set(s), t) | (t, Type::Set(s)) => Ok(Type::Set(Arc::from_iter(
                s.iter().cloned().chain(iter::once(t.clone())),
            ))),
            (u @ Type::Struct(t0), t @ Type::Struct(t1)) => {
                if t0.len() == t1.len() && t0 == t1 {
                    Ok(u.clone())
                } else {
                    Ok(Type::Set(Arc::from_iter([u.clone(), t.clone()])))
                }
            }
            (u @ Type::Struct(_), t) | (t, u @ Type::Struct(_)) => {
                Ok(Type::Set(Arc::from_iter([u.clone(), t.clone()])))
            }
            (u @ Type::Tuple(t0), t @ Type::Tuple(t1)) => {
                if t0 == t1 {
                    Ok(u.clone())
                } else {
                    Ok(Type::Set(Arc::from_iter([u.clone(), t.clone()])))
                }
            }
            (u @ Type::Tuple(_), t) | (t, u @ Type::Tuple(_)) => {
                Ok(Type::Set(Arc::from_iter([u.clone(), t.clone()])))
            }
            (u @ Type::Variant(tg0, t0), t @ Type::Variant(tg1, t1)) => {
                if tg0 == tg1 && t0.len() == t1.len() {
                    let typs = t0
                        .iter()
                        .zip(t1.iter())
                        .map(|(t0, t1)| t0.union_int(env, hist, t1))
                        .collect::<Result<SmallVec<[_; 8]>>>()?;
                    Ok(Type::Variant(tg0.clone(), Arc::from_iter(typs.into_iter())))
                } else {
                    Ok(Type::Set(Arc::from_iter([u.clone(), t.clone()])))
                }
            }
            (u @ Type::Variant(_, _), t) | (t, u @ Type::Variant(_, _)) => {
                Ok(Type::Set(Arc::from_iter([u.clone(), t.clone()])))
            }
            (Type::Fn(f0), Type::Fn(f1)) => {
                if f0 == f1 {
                    Ok(Type::Fn(f0.clone()))
                } else {
                    Ok(Type::Set(Arc::from_iter([
                        Type::Fn(f0.clone()),
                        Type::Fn(f1.clone()),
                    ])))
                }
            }
            (f @ Type::Fn(_), t) | (t, f @ Type::Fn(_)) => {
                Ok(Type::Set(Arc::from_iter([f.clone(), t.clone()])))
            }
            (t0 @ Type::TVar(_), t1 @ Type::TVar(_)) => {
                if t0 == t1 {
                    Ok(t0.clone())
                } else {
                    Ok(Type::Set(Arc::from_iter([t0.clone(), t1.clone()])))
                }
            }
            (t0 @ Type::TVar(_), t1) | (t1, t0 @ Type::TVar(_)) => {
                Ok(Type::Set(Arc::from_iter([t0.clone(), t1.clone()])))
            }
            (t @ Type::ByRef(_), u) | (u, t @ Type::ByRef(_)) => {
                Ok(Type::Set(Arc::from_iter([t.clone(), u.clone()])))
            }
        }
    }

    pub fn union<R: Rt, E: UserEvent>(&self, env: &Env<R, E>, t: &Self) -> Result<Self> {
        Ok(self.union_int(env, &mut LPooled::take(), t)?.normalize())
    }

    fn diff_int<R: Rt, E: UserEvent>(
        &self,
        env: &Env<R, E>,
        hist: &mut FxHashMap<(usize, usize), Type>,
        t: &Self,
    ) -> Result<Self> {
        match (self, t) {
            (
                Type::Ref { scope: s0, name: n0, .. },
                Type::Ref { scope: s1, name: n1, .. },
            ) if s0 == s1 && n0 == n1 => Ok(Type::Primitive(BitFlags::empty())),
            (t0 @ Type::Ref { .. }, t1) | (t0, t1 @ Type::Ref { .. }) => {
                let t0 = t0.lookup_ref(env)?;
                let t1 = t1.lookup_ref(env)?;
                let t0_addr = (t0 as *const Type).addr();
                let t1_addr = (t1 as *const Type).addr();
                match hist.get(&(t0_addr, t1_addr)) {
                    Some(r) => Ok(r.clone()),
                    None => {
                        let r = Type::Primitive(BitFlags::empty());
                        hist.insert((t0_addr, t1_addr), r);
                        match t0.diff_int(env, hist, &t1) {
                            Ok(r) => {
                                hist.insert((t0_addr, t1_addr), r.clone());
                                Ok(r)
                            }
                            Err(e) => {
                                hist.remove(&(t0_addr, t1_addr));
                                Err(e)
                            }
                        }
                    }
                }
            }
            (Type::Set(s0), Type::Set(s1)) => {
                let mut s: SmallVec<[Type; 4]> = smallvec![];
                for i in 0..s0.len() {
                    s.push(s0[i].clone());
                    for j in 0..s1.len() {
                        s[i] = s[i].diff_int(env, hist, &s1[j])?
                    }
                }
                Ok(Self::flatten_set(s.into_iter()))
            }
            (Type::Set(s), t) => Ok(Self::flatten_set(
                s.iter()
                    .map(|s| s.diff_int(env, hist, t))
                    .collect::<Result<SmallVec<[_; 8]>>>()?,
            )),
            (t, Type::Set(s)) => {
                let mut t = t.clone();
                for st in s.iter() {
                    t = t.diff_int(env, hist, st)?;
                }
                Ok(t)
            }
            (Type::Tuple(t0), Type::Tuple(t1)) => {
                if t0 == t1 {
                    Ok(Type::Primitive(BitFlags::empty()))
                } else {
                    Ok(self.clone())
                }
            }
            (Type::Struct(t0), Type::Struct(t1)) => {
                if t0.len() == t1.len() && t0 == t1 {
                    Ok(Type::Primitive(BitFlags::empty()))
                } else {
                    Ok(self.clone())
                }
            }
            (Type::Variant(tg0, t0), Type::Variant(tg1, t1)) => {
                if tg0 == tg1 && t0.len() == t1.len() && t0 == t1 {
                    Ok(Type::Primitive(BitFlags::empty()))
                } else {
                    Ok(self.clone())
                }
            }
            (Type::Map { key: k0, value: v0 }, Type::Map { key: k1, value: v1 }) => {
                if k0 == k1 && v0 == v1 {
                    Ok(Type::Primitive(BitFlags::empty()))
                } else {
                    Ok(self.clone())
                }
            }
            (Type::Map { .. }, Type::Primitive(p)) => {
                if p.contains(Typ::Map) {
                    Ok(Type::Primitive(BitFlags::empty()))
                } else {
                    Ok(self.clone())
                }
            }
            (Type::Primitive(p), Type::Map { key, value }) => {
                if **key == Type::Any && **value == Type::Any {
                    let mut p = *p;
                    p.remove(Typ::Map);
                    Ok(Type::Primitive(p))
                } else {
                    Ok(Type::Primitive(*p))
                }
            }
            (Type::Fn(f0), Type::Fn(f1)) => {
                if f0 == f1 {
                    Ok(Type::Primitive(BitFlags::empty()))
                } else {
                    Ok(Type::Fn(f0.clone()))
                }
            }
            (Type::TVar(tv0), Type::TVar(tv1)) => {
                if tv0.read().typ.as_ptr() == tv1.read().typ.as_ptr() {
                    return Ok(Type::Primitive(BitFlags::empty()));
                }
                Ok(match (&*tv0.read().typ.read(), &*tv1.read().typ.read()) {
                    (None, _) | (_, None) => Type::TVar(tv0.clone()),
                    (Some(t0), Some(t1)) => t0.diff_int(env, hist, t1)?,
                })
            }
            (Type::TVar(tv), t) => Ok(match &*tv.read().typ.read() {
                Some(tv) => tv.diff_int(env, hist, t)?,
                None => self.clone(),
            }),
            (t, Type::TVar(tv)) => Ok(match &*tv.read().typ.read() {
                Some(tv) => t.diff_int(env, hist, tv)?,
                None => self.clone(),
            }),
            (Type::Array(t0), Type::Array(t1)) => {
                if t0 == t1 {
                    Ok(Type::Primitive(BitFlags::empty()))
                } else {
                    Ok(Type::Array(Arc::new(t0.diff_int(env, hist, t1)?)))
                }
            }
            (Type::Primitive(p), Type::Array(t)) => {
                if &**t == &Type::Any {
                    let mut s = *p;
                    s.remove(Typ::Array);
                    Ok(Type::Primitive(s))
                } else {
                    Ok(Type::Primitive(*p))
                }
            }
            (
                Type::Array(_) | Type::Struct(_) | Type::Tuple(_) | Type::Variant(_, _),
                Type::Primitive(p),
            ) => {
                if p.contains(Typ::Array) {
                    Ok(Type::Primitive(BitFlags::empty()))
                } else {
                    Ok(self.clone())
                }
            }
            (_, Type::Any) => Ok(Type::Primitive(BitFlags::empty())),
            (Type::Any, _) => Ok(Type::Any),
            (Type::Primitive(s0), Type::Primitive(s1)) => {
                let mut s = *s0;
                s.remove(*s1);
                Ok(Type::Primitive(s))
            }
            (Type::Primitive(p), Type::Error(e)) => {
                if &**e == &Type::Any {
                    let mut s = *p;
                    s.remove(Typ::Error);
                    Ok(Type::Primitive(s))
                } else {
                    Ok(Type::Primitive(*p))
                }
            }
            (Type::Error(_), Type::Primitive(p)) => {
                if p.contains(Typ::Error) {
                    Ok(Type::Primitive(BitFlags::empty()))
                } else {
                    Ok(self.clone())
                }
            }
            (Type::Error(e0), Type::Error(e1)) => {
                if e0 == e1 {
                    Ok(Type::Primitive(BitFlags::empty()))
                } else {
                    Ok(Type::Error(Arc::new(e0.diff_int(env, hist, e1)?)))
                }
            }
            (Type::ByRef(t0), Type::ByRef(t1)) => {
                Ok(Type::ByRef(Arc::new(t0.diff_int(env, hist, t1)?)))
            }
            (Type::Fn(_), _)
            | (_, Type::Fn(_))
            | (Type::Array(_), _)
            | (_, Type::Array(_))
            | (Type::Tuple(_), _)
            | (_, Type::Tuple(_))
            | (Type::Struct(_), _)
            | (_, Type::Struct(_))
            | (Type::Variant(_, _), _)
            | (_, Type::Variant(_, _))
            | (Type::ByRef(_), _)
            | (_, Type::ByRef(_))
            | (Type::Error(_), _)
            | (_, Type::Error(_))
            | (Type::Primitive(_), _)
            | (_, Type::Primitive(_))
            | (Type::Bottom, _)
            | (Type::Map { .. }, _) => Ok(self.clone()),
        }
    }

    pub fn diff<R: Rt, E: UserEvent>(&self, env: &Env<R, E>, t: &Self) -> Result<Self> {
        Ok(self.diff_int(env, &mut LPooled::take(), t)?.normalize())
    }

    pub fn any() -> Self {
        Self::Any
    }

    pub fn boolean() -> Self {
        Self::Primitive(Typ::Bool.into())
    }

    pub fn number() -> Self {
        Self::Primitive(Typ::number())
    }

    pub fn int() -> Self {
        Self::Primitive(Typ::integer())
    }

    pub fn uint() -> Self {
        Self::Primitive(Typ::unsigned_integer())
    }

    /// alias type variables with the same name to each other
    pub fn alias_tvars(&self, known: &mut FxHashMap<ArcStr, TVar>) {
        match self {
            Type::Bottom | Type::Any | Type::Primitive(_) => (),
            Type::Ref { params, .. } => {
                for t in params.iter() {
                    t.alias_tvars(known);
                }
            }
            Type::Error(t) => t.alias_tvars(known),
            Type::Array(t) => t.alias_tvars(known),
            Type::Map { key, value } => {
                key.alias_tvars(known);
                value.alias_tvars(known);
            }
            Type::ByRef(t) => t.alias_tvars(known),
            Type::Tuple(ts) => {
                for t in ts.iter() {
                    t.alias_tvars(known)
                }
            }
            Type::Struct(ts) => {
                for (_, t) in ts.iter() {
                    t.alias_tvars(known)
                }
            }
            Type::Variant(_, ts) => {
                for t in ts.iter() {
                    t.alias_tvars(known)
                }
            }
            Type::TVar(tv) => match known.entry(tv.name.clone()) {
                Entry::Occupied(e) => {
                    let v = e.get();
                    v.freeze();
                    tv.alias(v);
                }
                Entry::Vacant(e) => {
                    e.insert(tv.clone());
                    ()
                }
            },
            Type::Fn(ft) => ft.alias_tvars(known),
            Type::Set(s) => {
                for typ in s.iter() {
                    typ.alias_tvars(known)
                }
            }
        }
    }

    pub fn collect_tvars(&self, known: &mut FxHashMap<ArcStr, TVar>) {
        match self {
            Type::Bottom | Type::Any | Type::Primitive(_) => (),
            Type::Ref { params, .. } => {
                for t in params.iter() {
                    t.collect_tvars(known);
                }
            }
            Type::Error(t) => t.collect_tvars(known),
            Type::Array(t) => t.collect_tvars(known),
            Type::Map { key, value } => {
                key.collect_tvars(known);
                value.collect_tvars(known);
            }
            Type::ByRef(t) => t.collect_tvars(known),
            Type::Tuple(ts) => {
                for t in ts.iter() {
                    t.collect_tvars(known)
                }
            }
            Type::Struct(ts) => {
                for (_, t) in ts.iter() {
                    t.collect_tvars(known)
                }
            }
            Type::Variant(_, ts) => {
                for t in ts.iter() {
                    t.collect_tvars(known)
                }
            }
            Type::TVar(tv) => match known.entry(tv.name.clone()) {
                Entry::Occupied(_) => (),
                Entry::Vacant(e) => {
                    e.insert(tv.clone());
                    ()
                }
            },
            Type::Fn(ft) => ft.collect_tvars(known),
            Type::Set(s) => {
                for typ in s.iter() {
                    typ.collect_tvars(known)
                }
            }
        }
    }

    pub fn check_tvars_declared(&self, declared: &FxHashSet<ArcStr>) -> Result<()> {
        match self {
            Type::Bottom | Type::Any | Type::Primitive(_) => Ok(()),
            Type::Ref { params, .. } => {
                params.iter().try_for_each(|t| t.check_tvars_declared(declared))
            }
            Type::Error(t) => t.check_tvars_declared(declared),
            Type::Array(t) => t.check_tvars_declared(declared),
            Type::Map { key, value } => {
                key.check_tvars_declared(declared)?;
                value.check_tvars_declared(declared)
            }
            Type::ByRef(t) => t.check_tvars_declared(declared),
            Type::Tuple(ts) => {
                ts.iter().try_for_each(|t| t.check_tvars_declared(declared))
            }
            Type::Struct(ts) => {
                ts.iter().try_for_each(|(_, t)| t.check_tvars_declared(declared))
            }
            Type::Variant(_, ts) => {
                ts.iter().try_for_each(|t| t.check_tvars_declared(declared))
            }
            Type::TVar(tv) => {
                if !declared.contains(&tv.name) {
                    bail!("undeclared type variable '{}'", tv.name)
                } else {
                    Ok(())
                }
            }
            Type::Set(s) => s.iter().try_for_each(|t| t.check_tvars_declared(declared)),
            Type::Fn(_) => Ok(()),
        }
    }

    pub fn has_unbound(&self) -> bool {
        match self {
            Type::Bottom | Type::Any | Type::Primitive(_) => false,
            Type::Ref { .. } => false,
            Type::Error(e) => e.has_unbound(),
            Type::Array(t0) => t0.has_unbound(),
            Type::Map { key, value } => key.has_unbound() || value.has_unbound(),
            Type::ByRef(t0) => t0.has_unbound(),
            Type::Tuple(ts) => ts.iter().any(|t| t.has_unbound()),
            Type::Struct(ts) => ts.iter().any(|(_, t)| t.has_unbound()),
            Type::Variant(_, ts) => ts.iter().any(|t| t.has_unbound()),
            Type::TVar(tv) => tv.read().typ.read().is_some(),
            Type::Set(s) => s.iter().any(|t| t.has_unbound()),
            Type::Fn(ft) => ft.has_unbound(),
        }
    }

    /// bind all unbound type variables to the specified type
    pub fn bind_as(&self, t: &Self) {
        match self {
            Type::Bottom | Type::Any | Type::Primitive(_) => (),
            Type::Ref { .. } => (),
            Type::Error(t0) => t0.bind_as(t),
            Type::Array(t0) => t0.bind_as(t),
            Type::Map { key, value } => {
                key.bind_as(t);
                value.bind_as(t);
            }
            Type::ByRef(t0) => t0.bind_as(t),
            Type::Tuple(ts) => {
                for elt in ts.iter() {
                    elt.bind_as(t)
                }
            }
            Type::Struct(ts) => {
                for (_, elt) in ts.iter() {
                    elt.bind_as(t)
                }
            }
            Type::Variant(_, ts) => {
                for elt in ts.iter() {
                    elt.bind_as(t)
                }
            }
            Type::TVar(tv) => {
                let tv = tv.read();
                let mut tv = tv.typ.write();
                if tv.is_none() {
                    *tv = Some(t.clone());
                }
            }
            Type::Set(s) => {
                for elt in s.iter() {
                    elt.bind_as(t)
                }
            }
            Type::Fn(ft) => ft.bind_as(t),
        }
    }

    /// return a copy of self with all type variables unbound and
    /// unaliased. self will not be modified
    pub fn reset_tvars(&self) -> Type {
        match self {
            Type::Bottom => Type::Bottom,
            Type::Any => Type::Any,
            Type::Primitive(p) => Type::Primitive(*p),
            Type::Ref { scope, name, params } => Type::Ref {
                scope: scope.clone(),
                name: name.clone(),
                params: Arc::from_iter(params.iter().map(|t| t.reset_tvars())),
            },
            Type::Error(t0) => Type::Error(Arc::new(t0.reset_tvars())),
            Type::Array(t0) => Type::Array(Arc::new(t0.reset_tvars())),
            Type::Map { key, value } => {
                let key = Arc::new(key.reset_tvars());
                let value = Arc::new(value.reset_tvars());
                Type::Map { key, value }
            }
            Type::ByRef(t0) => Type::ByRef(Arc::new(t0.reset_tvars())),
            Type::Tuple(ts) => {
                Type::Tuple(Arc::from_iter(ts.iter().map(|t| t.reset_tvars())))
            }
            Type::Struct(ts) => Type::Struct(Arc::from_iter(
                ts.iter().map(|(n, t)| (n.clone(), t.reset_tvars())),
            )),
            Type::Variant(tag, ts) => Type::Variant(
                tag.clone(),
                Arc::from_iter(ts.iter().map(|t| t.reset_tvars())),
            ),
            Type::TVar(tv) => Type::TVar(TVar::empty_named(tv.name.clone())),
            Type::Set(s) => Type::Set(Arc::from_iter(s.iter().map(|t| t.reset_tvars()))),
            Type::Fn(fntyp) => Type::Fn(Arc::new(fntyp.reset_tvars())),
        }
    }

    /// return a copy of self with every TVar named in known replaced
    /// with the corresponding type
    pub fn replace_tvars(&self, known: &FxHashMap<ArcStr, Self>) -> Type {
        match self {
            Type::TVar(tv) => match known.get(&tv.name) {
                Some(t) => t.clone(),
                None => Type::TVar(tv.clone()),
            },
            Type::Bottom => Type::Bottom,
            Type::Any => Type::Any,
            Type::Primitive(p) => Type::Primitive(*p),
            Type::Ref { scope, name, params } => Type::Ref {
                scope: scope.clone(),
                name: name.clone(),
                params: Arc::from_iter(params.iter().map(|t| t.replace_tvars(known))),
            },
            Type::Error(t0) => Type::Error(Arc::new(t0.replace_tvars(known))),
            Type::Array(t0) => Type::Array(Arc::new(t0.replace_tvars(known))),
            Type::Map { key, value } => {
                let key = Arc::new(key.replace_tvars(known));
                let value = Arc::new(value.replace_tvars(known));
                Type::Map { key, value }
            }
            Type::ByRef(t0) => Type::ByRef(Arc::new(t0.replace_tvars(known))),
            Type::Tuple(ts) => {
                Type::Tuple(Arc::from_iter(ts.iter().map(|t| t.replace_tvars(known))))
            }
            Type::Struct(ts) => Type::Struct(Arc::from_iter(
                ts.iter().map(|(n, t)| (n.clone(), t.replace_tvars(known))),
            )),
            Type::Variant(tag, ts) => Type::Variant(
                tag.clone(),
                Arc::from_iter(ts.iter().map(|t| t.replace_tvars(known))),
            ),
            Type::Set(s) => {
                Type::Set(Arc::from_iter(s.iter().map(|t| t.replace_tvars(known))))
            }
            Type::Fn(fntyp) => Type::Fn(Arc::new(fntyp.replace_tvars(known))),
        }
    }

    fn strip_error_int<R: Rt, E: UserEvent>(
        &self,
        env: &Env<R, E>,
        hist: &mut FxHashSet<usize>,
    ) -> Option<Type> {
        match self {
            Type::Error(t) => match t.strip_error_int(env, hist) {
                Some(t) => Some(t),
                None => Some((**t).clone()),
            },
            Type::TVar(tv) => {
                tv.read().typ.read().as_ref().and_then(|t| t.strip_error_int(env, hist))
            }
            Type::Primitive(p) => {
                if *p == BitFlags::from(Typ::Error) {
                    Some(Type::Any)
                } else {
                    None
                }
            }
            Type::Ref { .. } => {
                let t = self.lookup_ref(env).ok()?;
                let addr = t as *const Type as usize;
                if hist.insert(addr) {
                    t.strip_error_int(env, hist)
                } else {
                    None
                }
            }
            Type::Set(s) => {
                let r = Self::flatten_set(
                    s.iter().filter_map(|t| t.strip_error_int(env, hist)),
                );
                match r {
                    Type::Primitive(p) if p.is_empty() => None,
                    t => Some(t),
                }
            }
            Type::Array(_)
            | Type::Map { .. }
            | Type::ByRef(_)
            | Type::Tuple(_)
            | Type::Struct(_)
            | Type::Variant(_, _)
            | Type::Fn(_)
            | Type::Any
            | Type::Bottom => None,
        }
    }

    /// remove the outer error type and return the inner payload, fail if self
    /// isn't an error or contains non error types
    pub fn strip_error<R: Rt, E: UserEvent>(&self, env: &Env<R, E>) -> Option<Self> {
        self.strip_error_int(env, &mut LPooled::take())
    }

    /// Unbind any bound tvars, but do not unalias them.
    pub(crate) fn unbind_tvars(&self) {
        match self {
            Type::Bottom | Type::Any | Type::Primitive(_) | Type::Ref { .. } => (),
            Type::Error(t0) => t0.unbind_tvars(),
            Type::Array(t0) => t0.unbind_tvars(),
            Type::Map { key, value } => {
                key.unbind_tvars();
                value.unbind_tvars();
            }
            Type::ByRef(t0) => t0.unbind_tvars(),
            Type::Tuple(ts) | Type::Variant(_, ts) | Type::Set(ts) => {
                for t in ts.iter() {
                    t.unbind_tvars()
                }
            }
            Type::Struct(ts) => {
                for (_, t) in ts.iter() {
                    t.unbind_tvars()
                }
            }
            Type::TVar(tv) => tv.unbind(),
            Type::Fn(fntyp) => fntyp.unbind_tvars(),
        }
    }

    fn check_cast_int<R: Rt, E: UserEvent>(
        &self,
        env: &Env<R, E>,
        hist: &mut FxHashSet<usize>,
    ) -> Result<()> {
        match self {
            Type::Primitive(_) | Type::Any => Ok(()),
            Type::Fn(_) => bail!("can't cast a value to a function"),
            Type::Bottom => bail!("can't cast a value to bottom"),
            Type::Set(s) => Ok(for t in s.iter() {
                t.check_cast_int(env, hist)?
            }),
            Type::TVar(tv) => match &*tv.read().typ.read() {
                Some(t) => t.check_cast_int(env, hist),
                None => bail!("can't cast a value to a free type variable"),
            },
            Type::Error(e) => e.check_cast_int(env, hist),
            Type::Array(et) => et.check_cast_int(env, hist),
            Type::Map { key, value } => {
                key.check_cast_int(env, hist)?;
                value.check_cast_int(env, hist)
            }
            Type::ByRef(_) => bail!("can't cast a reference"),
            Type::Tuple(ts) => Ok(for t in ts.iter() {
                t.check_cast_int(env, hist)?
            }),
            Type::Struct(ts) => Ok(for (_, t) in ts.iter() {
                t.check_cast_int(env, hist)?
            }),
            Type::Variant(_, ts) => Ok(for t in ts.iter() {
                t.check_cast_int(env, hist)?
            }),
            Type::Ref { .. } => {
                let t = self.lookup_ref(env)?;
                let t_addr = (t as *const Type).addr();
                if hist.contains(&t_addr) {
                    Ok(())
                } else {
                    hist.insert(t_addr);
                    t.check_cast_int(env, hist)
                }
            }
        }
    }

    pub fn check_cast<R: Rt, E: UserEvent>(&self, env: &Env<R, E>) -> Result<()> {
        self.check_cast_int(env, &mut LPooled::take())
    }

    fn cast_value_int<R: Rt, E: UserEvent>(
        &self,
        env: &Env<R, E>,
        hist: &mut FxHashSet<usize>,
        v: Value,
    ) -> Result<Value> {
        if self.is_a_int(env, hist, &v) {
            return Ok(v);
        }
        match self {
            Type::Bottom => bail!("can't cast {v} to Bottom"),
            Type::Fn(_) => bail!("can't cast {v} to a function"),
            Type::ByRef(_) => bail!("can't cast {v} to a reference"),
            Type::Primitive(s) => s
                .iter()
                .find_map(|t| v.clone().cast(t))
                .ok_or_else(|| anyhow!("can't cast {v} to {self}")),
            Type::Any => Ok(v),
            Type::Error(e) => {
                let v = match v {
                    Value::Error(v) => (*v).clone(),
                    v => v,
                };
                Ok(Value::Error(Arc::new(e.cast_value_int(env, hist, v)?)))
            }
            Type::Array(et) => match v {
                Value::Array(elts) => {
                    let mut va = elts
                        .iter()
                        .map(|el| et.cast_value_int(env, hist, el.clone()))
                        .collect::<Result<LPooled<Vec<Value>>>>()?;
                    Ok(Value::Array(ValArray::from_iter_exact(va.drain(..))))
                }
                v => Ok(Value::Array([et.cast_value_int(env, hist, v)?].into())),
            },
            Type::Map { key, value } => match v {
                Value::Map(m) => {
                    let mut m = m
                        .into_iter()
                        .map(|(k, v)| {
                            Ok((
                                key.cast_value_int(env, hist, k.clone())?,
                                value.cast_value_int(env, hist, v.clone())?,
                            ))
                        })
                        .collect::<Result<LPooled<Vec<(Value, Value)>>>>()?;
                    Ok(Value::Map(Map::from_iter(m.drain(..))))
                }
                Value::Array(a) => {
                    let mut m = a
                        .iter()
                        .map(|a| match a {
                            Value::Array(a) if a.len() == 2 => Ok((
                                key.cast_value_int(env, hist, a[0].clone())?,
                                value.cast_value_int(env, hist, a[1].clone())?,
                            )),
                            _ => bail!("expected an array of pairs"),
                        })
                        .collect::<Result<LPooled<Vec<(Value, Value)>>>>()?;
                    Ok(Value::Map(Map::from_iter(m.drain(..))))
                }
                _ => bail!("can't cast {v} to {self}"),
            },
            Type::Tuple(ts) => match v {
                Value::Array(elts) => {
                    if elts.len() != ts.len() {
                        bail!("tuple size mismatch {self} with {}", Value::Array(elts))
                    }
                    let a = ts
                        .iter()
                        .zip(elts.iter())
                        .map(|(t, el)| t.cast_value_int(env, hist, el.clone()))
                        .collect::<Result<SmallVec<[Value; 8]>>>()?;
                    Ok(Value::Array(ValArray::from_iter_exact(a.into_iter())))
                }
                v => bail!("can't cast {v} to {self}"),
            },
            Type::Struct(ts) => match v {
                Value::Array(elts) => {
                    if elts.len() != ts.len() {
                        bail!("struct size mismatch {self} with {}", Value::Array(elts))
                    }
                    let is_pairs = elts.iter().all(|v| match v {
                        Value::Array(a) if a.len() == 2 => match &a[0] {
                            Value::String(_) => true,
                            _ => false,
                        },
                        _ => false,
                    });
                    if !is_pairs {
                        bail!("expected array of pairs, got {}", Value::Array(elts))
                    }
                    let mut elts_s: SmallVec<[&Value; 16]> = elts.iter().collect();
                    elts_s.sort_by_key(|v| match v {
                        Value::Array(a) => match &a[0] {
                            Value::String(s) => s,
                            _ => unreachable!(),
                        },
                        _ => unreachable!(),
                    });
                    let (keys_ok, ok) = ts.iter().zip(elts_s.iter()).fold(
                        Ok((true, true)),
                        |acc: Result<_>, ((fname, t), v)| {
                            let (kok, ok) = acc?;
                            let (name, v) = match v {
                                Value::Array(a) => match (&a[0], &a[1]) {
                                    (Value::String(n), v) => (n, v),
                                    _ => unreachable!(),
                                },
                                _ => unreachable!(),
                            };
                            Ok((
                                kok && name == fname,
                                ok && kok
                                    && t.contains(
                                        env,
                                        &Type::Primitive(Typ::get(v).into()),
                                    )?,
                            ))
                        },
                    )?;
                    if ok {
                        drop(elts_s);
                        return Ok(Value::Array(elts));
                    } else if keys_ok {
                        let elts = ts
                            .iter()
                            .zip(elts_s.iter())
                            .map(|((n, t), v)| match v {
                                Value::Array(a) => {
                                    let a = [
                                        Value::String(n.clone()),
                                        t.cast_value_int(env, hist, a[1].clone())?,
                                    ];
                                    Ok(Value::Array(ValArray::from_iter_exact(
                                        a.into_iter(),
                                    )))
                                }
                                _ => unreachable!(),
                            })
                            .collect::<Result<SmallVec<[Value; 8]>>>()?;
                        Ok(Value::Array(ValArray::from_iter_exact(elts.into_iter())))
                    } else {
                        drop(elts_s);
                        bail!("struct fields mismatch {self}, {}", Value::Array(elts))
                    }
                }
                v => bail!("can't cast {v} to {self}"),
            },
            Type::Variant(tag, ts) if ts.len() == 0 => match &v {
                Value::String(s) if s == tag => Ok(v),
                _ => bail!("variant tag mismatch expected {tag} got {v}"),
            },
            Type::Variant(tag, ts) => match &v {
                Value::Array(elts) => {
                    if ts.len() + 1 == elts.len() {
                        match &elts[0] {
                            Value::String(s) if s == tag => (),
                            v => bail!("variant tag mismatch expected {tag} got {v}"),
                        }
                        let a = iter::once(&Type::Primitive(Typ::String.into()))
                            .chain(ts.iter())
                            .zip(elts.iter())
                            .map(|(t, v)| t.cast_value_int(env, hist, v.clone()))
                            .collect::<Result<SmallVec<[Value; 8]>>>()?;
                        Ok(Value::Array(ValArray::from_iter_exact(a.into_iter())))
                    } else if ts.len() == elts.len() {
                        let mut a = ts
                            .iter()
                            .zip(elts.iter())
                            .map(|(t, v)| t.cast_value_int(env, hist, v.clone()))
                            .collect::<Result<SmallVec<[Value; 8]>>>()?;
                        a.insert(0, Value::String(tag.clone()));
                        Ok(Value::Array(ValArray::from_iter_exact(a.into_iter())))
                    } else {
                        bail!("variant length mismatch")
                    }
                }
                v => bail!("can't cast {v} to {self}"),
            },
            Type::Ref { .. } => self.lookup_ref(env)?.cast_value_int(env, hist, v),
            Type::Set(ts) => ts
                .iter()
                .find_map(|t| t.cast_value_int(env, hist, v.clone()).ok())
                .ok_or_else(|| anyhow!("can't cast {v} to {self}")),
            Type::TVar(tv) => match &*tv.read().typ.read() {
                Some(t) => t.cast_value_int(env, hist, v.clone()),
                None => Ok(v),
            },
        }
    }

    pub fn cast_value<R: Rt, E: UserEvent>(&self, env: &Env<R, E>, v: Value) -> Value {
        match self.cast_value_int(env, &mut LPooled::take(), v) {
            Ok(v) => v,
            Err(e) => Value::error(e.to_string()),
        }
    }

    fn is_a_int<R: Rt, E: UserEvent>(
        &self,
        env: &Env<R, E>,
        hist: &mut FxHashSet<usize>,
        v: &Value,
    ) -> bool {
        match self {
            Type::Ref { .. } => match self.lookup_ref(env) {
                Err(_) => false,
                Ok(t) => {
                    let t_addr = (t as *const Type).addr();
                    !hist.contains(&t_addr) && {
                        hist.insert(t_addr);
                        t.is_a_int(env, hist, v)
                    }
                }
            },
            Type::Primitive(t) => t.contains(Typ::get(&v)),
            Type::Any => true,
            Type::Array(et) => match v {
                Value::Array(a) => a.iter().all(|v| et.is_a_int(env, hist, v)),
                _ => false,
            },
            Type::Map { key, value } => match v {
                Value::Map(m) => m.into_iter().all(|(k, v)| {
                    key.is_a_int(env, hist, k) && value.is_a_int(env, hist, v)
                }),
                _ => false,
            },
            Type::Error(e) => match v {
                Value::Error(v) => e.is_a_int(env, hist, v),
                _ => false,
            },
            Type::ByRef(_) => matches!(v, Value::U64(_) | Value::V64(_)),
            Type::Tuple(ts) => match v {
                Value::Array(elts) => {
                    elts.len() == ts.len()
                        && ts
                            .iter()
                            .zip(elts.iter())
                            .all(|(t, v)| t.is_a_int(env, hist, v))
                }
                _ => false,
            },
            Type::Struct(ts) => match v {
                Value::Array(elts) => {
                    elts.len() == ts.len()
                        && ts.iter().zip(elts.iter()).all(|((n, t), v)| match v {
                            Value::Array(a) if a.len() == 2 => match &a[..] {
                                [Value::String(key), v] => {
                                    n == key && t.is_a_int(env, hist, v)
                                }
                                _ => false,
                            },
                            _ => false,
                        })
                }
                _ => false,
            },
            Type::Variant(tag, ts) if ts.len() == 0 => match &v {
                Value::String(s) => s == tag,
                _ => false,
            },
            Type::Variant(tag, ts) => match &v {
                Value::Array(elts) => {
                    ts.len() + 1 == elts.len()
                        && match &elts[0] {
                            Value::String(s) => s == tag,
                            _ => false,
                        }
                        && ts
                            .iter()
                            .zip(elts[1..].iter())
                            .all(|(t, v)| t.is_a_int(env, hist, v))
                }
                _ => false,
            },
            Type::TVar(tv) => match &*tv.read().typ.read() {
                None => true,
                Some(t) => t.is_a_int(env, hist, v),
            },
            Type::Fn(_) => match v {
                Value::U64(_) => true,
                _ => false,
            },
            Type::Bottom => true,
            Type::Set(ts) => ts.iter().any(|t| t.is_a_int(env, hist, v)),
        }
    }

    /// return true if v is structurally compatible with the type
    pub fn is_a<R: Rt, E: UserEvent>(&self, env: &Env<R, E>, v: &Value) -> bool {
        self.is_a_int(env, &mut LPooled::take(), v)
    }

    pub fn is_bot(&self) -> bool {
        match self {
            Type::Bottom => true,
            Type::Any
            | Type::TVar(_)
            | Type::Primitive(_)
            | Type::Ref { .. }
            | Type::Fn(_)
            | Type::Error(_)
            | Type::Array(_)
            | Type::ByRef(_)
            | Type::Tuple(_)
            | Type::Struct(_)
            | Type::Variant(_, _)
            | Type::Set(_)
            | Type::Map { .. } => false,
        }
    }

    pub fn with_deref<R, F: FnOnce(Option<&Self>) -> R>(&self, f: F) -> R {
        match self {
            Self::Bottom
            | Self::Any
            | Self::Primitive(_)
            | Self::Fn(_)
            | Self::Set(_)
            | Self::Error(_)
            | Self::Array(_)
            | Self::ByRef(_)
            | Self::Tuple(_)
            | Self::Struct(_)
            | Self::Variant(_, _)
            | Self::Ref { .. }
            | Self::Map { .. } => f(Some(self)),
            Self::TVar(tv) => f(tv.read().typ.read().as_ref()),
        }
    }

    pub(crate) fn flatten_set(set: impl IntoIterator<Item = Self>) -> Self {
        let init: Box<dyn Iterator<Item = Self>> = Box::new(set.into_iter());
        let mut iters: LPooled<Vec<Box<dyn Iterator<Item = Self>>>> =
            LPooled::from_iter([init]);
        let mut acc: LPooled<Vec<Self>> = LPooled::take();
        loop {
            match iters.last_mut() {
                None => break,
                Some(iter) => match iter.next() {
                    None => {
                        iters.pop();
                    }
                    Some(Type::Set(s)) => {
                        let v: SmallVec<[Self; 16]> =
                            s.iter().map(|t| t.clone()).collect();
                        iters.push(Box::new(v.into_iter()))
                    }
                    Some(Type::Any) => return Type::Any,
                    Some(t) => {
                        acc.push(t);
                        let mut i = 0;
                        let mut j = 0;
                        while i < acc.len() {
                            while j < acc.len() {
                                if j == i {
                                    j += 1;
                                    continue;
                                }
                                match acc[i].merge(&acc[j]) {
                                    None => j += 1,
                                    Some(t) => {
                                        acc[i] = t;
                                        acc.remove(j);
                                        i = 0;
                                        j = 0;
                                    }
                                }
                            }
                            i += 1;
                            j = 0;
                        }
                    }
                },
            }
        }
        acc.sort();
        match &**acc {
            [] => Type::Primitive(BitFlags::empty()),
            [t] => t.clone(),
            _ => Type::Set(Arc::from_iter(acc.drain(..))),
        }
    }

    pub(crate) fn normalize(&self) -> Self {
        match self {
            Type::Bottom | Type::Any | Type::Primitive(_) => self.clone(),
            Type::Ref { scope, name, params } => {
                let params = Arc::from_iter(params.iter().map(|t| t.normalize()));
                Type::Ref { scope: scope.clone(), name: name.clone(), params }
            }
            Type::TVar(tv) => Type::TVar(tv.normalize()),
            Type::Set(s) => Self::flatten_set(s.iter().map(|t| t.normalize())),
            Type::Error(t) => Type::Error(Arc::new(t.normalize())),
            Type::Array(t) => Type::Array(Arc::new(t.normalize())),
            Type::Map { key, value } => {
                let key = Arc::new(key.normalize());
                let value = Arc::new(value.normalize());
                Type::Map { key, value }
            }
            Type::ByRef(t) => Type::ByRef(Arc::new(t.normalize())),
            Type::Tuple(t) => {
                Type::Tuple(Arc::from_iter(t.iter().map(|t| t.normalize())))
            }
            Type::Struct(t) => Type::Struct(Arc::from_iter(
                t.iter().map(|(n, t)| (n.clone(), t.normalize())),
            )),
            Type::Variant(tag, t) => Type::Variant(
                tag.clone(),
                Arc::from_iter(t.iter().map(|t| t.normalize())),
            ),
            Type::Fn(ft) => Type::Fn(Arc::new(ft.normalize())),
        }
    }

    fn merge(&self, t: &Self) -> Option<Self> {
        macro_rules! flatten {
            ($t:expr) => {
                match $t {
                    Type::Set(et) => Self::flatten_set(et.iter().cloned()),
                    t => t.clone(),
                }
            };
        }
        match (self, t) {
            (
                Type::Ref { scope: s0, name: r0, params: a0 },
                Type::Ref { scope: s1, name: r1, params: a1 },
            ) => {
                if s0 == s1 && r0 == r1 && a0 == a1 {
                    Some(Type::Ref {
                        scope: s0.clone(),
                        name: r0.clone(),
                        params: a0.clone(),
                    })
                } else {
                    None
                }
            }
            (Type::Ref { .. }, _) | (_, Type::Ref { .. }) => None,
            (Type::Bottom, t) | (t, Type::Bottom) => Some(t.clone()),
            (Type::Any, _) | (_, Type::Any) => Some(Type::Any),
            (Type::Primitive(s0), Type::Primitive(s1)) => {
                Some(Type::Primitive(*s0 | *s1))
            }
            (Type::Primitive(p), t) | (t, Type::Primitive(p)) if p.is_empty() => {
                Some(t.clone())
            }
            (Type::Fn(f0), Type::Fn(f1)) => {
                if f0 == f1 {
                    Some(Type::Fn(f0.clone()))
                } else {
                    None
                }
            }
            (Type::Array(t0), Type::Array(t1)) => {
                if flatten!(&**t0) == flatten!(&**t1) {
                    Some(Type::Array(t0.clone()))
                } else {
                    None
                }
            }
            (Type::Primitive(p), Type::Array(_))
            | (Type::Array(_), Type::Primitive(p)) => {
                if p.contains(Typ::Array) {
                    Some(Type::Primitive(*p))
                } else {
                    None
                }
            }
            (Type::Map { key: k0, value: v0 }, Type::Map { key: k1, value: v1 }) => {
                if flatten!(&**k0) == flatten!(&**k1)
                    && flatten!(&**v0) == flatten!(&**v1)
                {
                    Some(Type::Map { key: k0.clone(), value: v0.clone() })
                } else {
                    None
                }
            }
            (Type::Error(t0), Type::Error(t1)) => {
                if flatten!(&**t0) == flatten!(&**t1) {
                    Some(Type::Error(t0.clone()))
                } else {
                    None
                }
            }
            (Type::ByRef(t0), Type::ByRef(t1)) => {
                t0.merge(t1).map(|t| Type::ByRef(Arc::new(t)))
            }
            (Type::Set(s0), Type::Set(s1)) => {
                Some(Self::flatten_set(s0.iter().cloned().chain(s1.iter().cloned())))
            }
            (Type::Set(s), Type::Primitive(p)) | (Type::Primitive(p), Type::Set(s))
                if p.is_empty() =>
            {
                Some(Type::Set(s.clone()))
            }
            (Type::Set(s), t) | (t, Type::Set(s)) => {
                Some(Self::flatten_set(s.iter().cloned().chain(iter::once(t.clone()))))
            }
            (Type::Tuple(t0), Type::Tuple(t1)) => {
                if t0.len() == t1.len() {
                    let t = t0
                        .iter()
                        .zip(t1.iter())
                        .map(|(t0, t1)| t0.merge(t1))
                        .collect::<Option<SmallVec<[Type; 8]>>>()?;
                    Some(Type::Tuple(Arc::from_iter(t)))
                } else {
                    None
                }
            }
            (Type::Variant(tag0, t0), Type::Variant(tag1, t1)) => {
                if tag0 == tag1 && t0.len() == t1.len() {
                    let t = t0
                        .iter()
                        .zip(t1.iter())
                        .map(|(t0, t1)| t0.merge(t1))
                        .collect::<Option<SmallVec<[Type; 8]>>>()?;
                    Some(Type::Variant(tag0.clone(), Arc::from_iter(t)))
                } else {
                    None
                }
            }
            (Type::Struct(t0), Type::Struct(t1)) => {
                if t0.len() == t1.len() {
                    let t = t0
                        .iter()
                        .zip(t1.iter())
                        .map(|((n0, t0), (n1, t1))| {
                            if n0 != n1 {
                                None
                            } else {
                                t0.merge(t1).map(|t| (n0.clone(), t))
                            }
                        })
                        .collect::<Option<SmallVec<[(ArcStr, Type); 8]>>>()?;
                    Some(Type::Struct(Arc::from_iter(t)))
                } else {
                    None
                }
            }
            (Type::TVar(tv0), Type::TVar(tv1)) if tv0.name == tv1.name && tv0 == tv1 => {
                Some(Type::TVar(tv0.clone()))
            }
            (Type::TVar(tv), t) => {
                tv.read().typ.read().as_ref().and_then(|tv| tv.merge(t))
            }
            (t, Type::TVar(tv)) => {
                tv.read().typ.read().as_ref().and_then(|tv| t.merge(tv))
            }
            (Type::ByRef(_), _)
            | (_, Type::ByRef(_))
            | (Type::Array(_), _)
            | (_, Type::Array(_))
            | (_, Type::Map { .. })
            | (Type::Map { .. }, _)
            | (Type::Tuple(_), _)
            | (_, Type::Tuple(_))
            | (Type::Struct(_), _)
            | (_, Type::Struct(_))
            | (Type::Variant(_, _), _)
            | (_, Type::Variant(_, _))
            | (_, Type::Fn(_))
            | (Type::Fn(_), _)
            | (Type::Error(_), _)
            | (_, Type::Error(_)) => None,
        }
    }

    pub fn scope_refs(&self, scope: &ModPath) -> Type {
        match self {
            Type::Bottom => Type::Bottom,
            Type::Any => Type::Any,
            Type::Primitive(s) => Type::Primitive(*s),
            Type::Error(t0) => Type::Error(Arc::new(t0.scope_refs(scope))),
            Type::Array(t0) => Type::Array(Arc::new(t0.scope_refs(scope))),
            Type::Map { key, value } => {
                let key = Arc::new(key.scope_refs(scope));
                let value = Arc::new(value.scope_refs(scope));
                Type::Map { key, value }
            }
            Type::ByRef(t) => Type::ByRef(Arc::new(t.scope_refs(scope))),
            Type::Tuple(ts) => {
                let i = ts.iter().map(|t| t.scope_refs(scope));
                Type::Tuple(Arc::from_iter(i))
            }
            Type::Variant(tag, ts) => {
                let i = ts.iter().map(|t| t.scope_refs(scope));
                Type::Variant(tag.clone(), Arc::from_iter(i))
            }
            Type::Struct(ts) => {
                let i = ts.iter().map(|(n, t)| (n.clone(), t.scope_refs(scope)));
                Type::Struct(Arc::from_iter(i))
            }
            Type::TVar(tv) => match tv.read().typ.read().as_ref() {
                None => Type::TVar(TVar::empty_named(tv.name.clone())),
                Some(typ) => {
                    let typ = typ.scope_refs(scope);
                    Type::TVar(TVar::named(tv.name.clone(), typ))
                }
            },
            Type::Ref { scope: _, name, params } => {
                let params = Arc::from_iter(params.iter().map(|t| t.scope_refs(scope)));
                Type::Ref { scope: scope.clone(), name: name.clone(), params }
            }
            Type::Set(ts) => {
                Type::Set(Arc::from_iter(ts.iter().map(|t| t.scope_refs(scope))))
            }
            Type::Fn(f) => Type::Fn(Arc::new(f.scope_refs(scope))),
        }
    }
}

impl fmt::Display for Type {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Self::Bottom => write!(f, "_"),
            Self::Any => write!(f, "Any"),
            Self::Ref { scope: _, name, params } => {
                write!(f, "{name}")?;
                if !params.is_empty() {
                    write!(f, "<")?;
                    for (i, t) in params.iter().enumerate() {
                        write!(f, "{t}")?;
                        if i < params.len() - 1 {
                            write!(f, ", ")?;
                        }
                    }
                    write!(f, ">")?;
                }
                Ok(())
            }
            Self::TVar(tv) => write!(f, "{tv}"),
            Self::Fn(t) => write!(f, "{t}"),
            Self::Error(t) => write!(f, "Error<{t}>"),
            Self::Array(t) => write!(f, "Array<{t}>"),
            Self::Map { key, value } => write!(f, "Map<{key}, {value}>"),
            Self::ByRef(t) => write!(f, "&{t}"),
            Self::Tuple(ts) => {
                write!(f, "(")?;
                for (i, t) in ts.iter().enumerate() {
                    write!(f, "{t}")?;
                    if i < ts.len() - 1 {
                        write!(f, ", ")?;
                    }
                }
                write!(f, ")")
            }
            Self::Variant(tag, ts) if ts.len() == 0 => {
                write!(f, "`{tag}")
            }
            Self::Variant(tag, ts) => {
                write!(f, "`{tag}(")?;
                for (i, t) in ts.iter().enumerate() {
                    write!(f, "{t}")?;
                    if i < ts.len() - 1 {
                        write!(f, ", ")?
                    }
                }
                write!(f, ")")
            }
            Self::Struct(ts) => {
                write!(f, "{{")?;
                for (i, (n, t)) in ts.iter().enumerate() {
                    write!(f, "{n}: {t}")?;
                    if i < ts.len() - 1 {
                        write!(f, ", ")?
                    }
                }
                write!(f, "}}")
            }
            Self::Set(s) => {
                write!(f, "[")?;
                for (i, t) in s.iter().enumerate() {
                    write!(f, "{t}")?;
                    if i < s.len() - 1 {
                        write!(f, ", ")?;
                    }
                }
                write!(f, "]")
            }
            Self::Primitive(s) => {
                let replace = PRINT_FLAGS.get().contains(PrintFlag::ReplacePrims);
                if replace && *s == Typ::number() {
                    write!(f, "Number")
                } else if replace && *s == Typ::float() {
                    write!(f, "Float")
                } else if replace && *s == Typ::real() {
                    write!(f, "Real")
                } else if replace && *s == Typ::integer() {
                    write!(f, "Int")
                } else if replace && *s == Typ::unsigned_integer() {
                    write!(f, "Uint")
                } else if replace && *s == Typ::signed_integer() {
                    write!(f, "Sint")
                } else if s.len() == 0 {
                    write!(f, "[]")
                } else if s.len() == 1 {
                    write!(f, "{}", s.iter().next().unwrap())
                } else {
                    let mut s = *s;
                    macro_rules! builtin {
                        ($set:expr, $name:literal) => {
                            if replace && s.contains($set) {
                                s.remove($set);
                                write!(f, $name)?;
                                if !s.is_empty() {
                                    write!(f, ", ")?
                                }
                            }
                        };
                    }
                    write!(f, "[")?;
                    builtin!(Typ::number(), "Number");
                    builtin!(Typ::real(), "Real");
                    builtin!(Typ::float(), "Float");
                    builtin!(Typ::integer(), "Int");
                    builtin!(Typ::unsigned_integer(), "Uint");
                    builtin!(Typ::signed_integer(), "Sint");
                    for (i, t) in s.iter().enumerate() {
                        write!(f, "{t}")?;
                        if i < s.len() - 1 {
                            write!(f, ", ")?;
                        }
                    }
                    write!(f, "]")
                }
            }
        }
    }
}