sui_eval/

lib.rs

//! Clean-room Nix language evaluator.
//!
//! Architecture:
//! Nix source → rnix parser (CST) → Evaluator (values)
//!
//! Parsing is delegated to the `rnix` crate (MIT).

use std::rc::Rc;

/// Core Nix builtins (90+ functions).
pub mod builtins;
/// Bidirectional conversion between bytecode VM values and tree-walker values.
pub mod convert;
/// Tree-walking evaluator using rnix's typed AST.
pub mod eval;
/// Content-addressed derivation path cache (redb-backed).
pub mod drv_cache;
/// Content-addressed evaluation cache (file hash + lock hash → result).
pub mod eval_cache;
/// Content-addressed input fetcher for flake.lock resolved inputs.
pub mod fetcher;
/// Native flake lock management — update, check, write.
pub mod flake_lock;
/// Pure-Rust git operations via gix/gitoxide (no CLI spawning, no C deps).
pub mod git;
/// Centralized path resolution (normalize, resolve relative, import).
pub mod path;
/// Source positions for `builtins.unsafeGetAttrPos` / `__curPos`.
pub mod pos;
/// Lightweight evaluation profiling counters.
pub mod perf;
/// ENV-RESOLVE M0 flag + per-source resolution-table plumbing (the
/// tree-walker's consume side of the `sui-resolve` side-table).
pub mod resolve_env;
/// Infinite recursion debugging tools (force chain, trace, depth limit, stats).
pub mod trace;
/// Nix value types, environments, thunks, and error types.
pub mod value;
/// Lazy evaluation primitives — making accidental eagerness impossible.
pub mod lazy;
/// Import-from-derivation: realize a derivation output mid-eval via a
/// binary-installed hook (the pure evaluator owns no build pipeline).
pub mod realize;

/// The normalized differential render (deep-forcing, error-propagating) the
/// sui↔sui differential + the `SUI_IR` shadow-eval latch byte-compare against
/// `eval_ir`. Must stay format-locked with `sui_ir::render`.
pub mod render;

/// Re-export flake lock types from sui-compat where they canonically live.
pub mod flake {
    pub use sui_compat::flake::*;
}

/// Evaluate a Nix expression string (convenience re-export).
pub use eval::{eval, eval_with_file};
/// Re-exported for ergonomic access from dependent crates.
pub use value::{EvalError, Value};

/// The evaluator trait — enables swapping evaluation strategies.
///
/// Implementations: tree-walking (current), bytecode VM (future),
/// delegation to external `nix eval` (fallback during transition).
pub trait Evaluator {
    /// Evaluate a Nix expression string.
    fn eval_expr(&self, input: &str) -> Result<Value, EvalError>;

    /// Evaluate a Nix file.
    fn eval_file(&self, path: &std::path::Path) -> Result<Value, EvalError>;
}

/// The default tree-walking evaluator.
pub struct TreeWalkEvaluator;

impl Evaluator for TreeWalkEvaluator {
    fn eval_expr(&self, input: &str) -> Result<Value, EvalError> {
        eval(input)
    }

    fn eval_file(&self, path: &std::path::Path) -> Result<Value, EvalError> {
        let source = std::fs::read_to_string(path)
            .map_err(|e| EvalError::IoError {
                context: format!("eval_file: {}", path.display()),
                message: e.to_string(),
            })?;
        let path_buf = path.to_path_buf();
        let _guard = eval::push_eval_file(path_buf.clone());
        eval::eval_with_file(&source, Some(path_buf))
    }
}

/// Bytecode VM evaluator — compiles to bytecode and executes on the stack VM.
///
/// Installs a flake resolver that delegates `builtins.getFlake` to the
/// tree-walker's [`builtins::evaluate_flake`], so the VM gets correct
/// flake input resolution for all input types (GitHub, path, indirect).
pub struct BytecodeEvaluator;

/// RAII bundle of every tree-walker bridge the VM needs, held for the
/// duration of one VM evaluation.
///
/// Dropping this restores whatever was installed before.
pub struct VmBridgeGuards {
    _flake: sui_bytecode::FlakeResolverGuard,
    _bridge: sui_bytecode::BuiltinBridgeGuard,
    _path: sui_bytecode::PathMaterializerGuard,
}

/// Install every tree-walker bridge the bytecode VM depends on, returning an
/// RAII bundle that uninstalls them on drop.
///
/// ★ THIS IS THE ONE INSTALL SITE. `BytecodeEvaluator` calls it, and so does
/// `sui-bytecode`'s bridged-parity test suite — so a test can never drift from
/// what production wires up (which is precisely how a bridge-dependent
/// divergence stayed invisible: `sui-bytecode/tests/parity.rs` runs the VM with
/// NO bridge installed).
///
/// Three bridges, all thread-local (`sui-bytecode` cannot depend on `sui-eval`
/// — see that crate's `Cargo.toml` for the publish cycle):
/// 1. **Flake resolver** — delegates `builtins.getFlake` to the tree-walker.
/// 2. **Builtin bridge** — delegates builtins the VM has no native
///    implementation for (`getEnv`, `match`, `split`, `fromTOML`,
///    `genericClosure`, `readDir`, `hashFile`, …) to the tree-walker.
/// 3. **Path materializer** — redirects the VM's filesystem reads through
///    [`path::materialize`], so a flake input's `/nix/store/<narhash>-source`
///    path (which sui never writes to disk) resolves to the real fetcher-cache
///    tree. Without it the VM answers `pathExists = false` for every file in a
///    fetched input — silently, since `false` is a legal answer and the VM's
///    per-file fallback only fires on an ERROR.
#[must_use]
pub fn install_vm_bridges() -> VmBridgeGuards {
    // Install flake resolver: tree-walker evaluate_flake → StringKeyedValue
    let _flake_guard = sui_bytecode::set_flake_resolver(Box::new(|flake_ref: &str| {
            let flake_dir = if flake_ref.starts_with('/') || flake_ref.starts_with('.') {
                std::path::PathBuf::from(flake_ref)
            } else if let Some(path) = flake_ref.strip_prefix("path:") {
                std::path::PathBuf::from(path)
            } else {
                return Err(format!("unsupported flake reference: {flake_ref}"));
            };

            let result = builtins::evaluate_flake(&flake_dir)
                .map_err(|e| e.to_string())?;

            // Convert tree-walker Value → StringKeyedValue for the VM.
            Ok(eval_to_string_keyed(&result))
        }));

        // Install builtin bridge: VM builtins → tree-walker builtins
        let _bridge_guard = sui_bytecode::set_builtin_bridge(Box::new(
            |name: &str, args: Vec<sui_bytecode::StringKeyedValue>| {
                // Special case: __import — the VM compiler couldn't handle
                // this file, so fall back to the tree-walker evaluator.
                if name == "__import" {
                    let path_str = match &args[0] {
                        sui_bytecode::StringKeyedValue::Path(p)
                        | sui_bytecode::StringKeyedValue::String(p) => p.clone(),
                        _ => return Err("__import: expected path or string argument".to_string()),
                    };
                    let path = std::path::Path::new(&path_str);
                    let source = std::fs::read_to_string(path)
                        .map_err(|e| format!("__import: {}: {e}", path.display()))?;
                    let path_buf = path.to_path_buf();
                    let _guard = eval::push_eval_file(path_buf.clone());
                    let result = eval::eval_with_file(&source, Some(path_buf))
                        .map_err(|e| e.to_string())?;
                    // Force the top-level result before converting — if the
                    // tree-walker returned a thunk, the VM would see
                    // "expected set, got thunk" when accessing attrs.
                    let forced = eval::force_value(&result)
                        .map_err(|e| e.to_string())?;
                    return Ok(eval_to_string_keyed(&forced));
                }

                // Convert StringKeyedValue args → tree-walker Value
                let eval_args: Vec<Value> = args
                    .iter()
                    .map(|a| convert::string_keyed_to_eval(a))
                    .collect();

                // Call the tree-walker builtin
                let result = builtins::call_builtin_by_name(name, &eval_args)
                    .map_err(|e| e.to_string())?;

                // Force the result before converting — builtins may return
                // thunks that the VM cannot handle directly.
                let forced = eval::force_value(&result)
                    .map_err(|e| e.to_string())?;

                // Convert tree-walker Value → StringKeyedValue
                Ok(eval_to_string_keyed(&forced))
            },
        ));

    // Install path materializer: the VM's `std::fs` reads (pathExists,
    // readFile, readFileType, import, scopedImport) go through the SAME
    // store-path→cache-dir redirect the tree-walker uses. Identity for any
    // path that is not under a registered flake-input source, so this is a
    // no-op for every ordinary path.
    let _path_guard = sui_bytecode::set_path_materializer(Box::new(|p: &str| {
        crate::path::materialize_str(p)
    }));

    VmBridgeGuards {
        _flake: _flake_guard,
        _bridge: _bridge_guard,
        _path: _path_guard,
    }
}

impl BytecodeEvaluator {
    /// Run a bytecode evaluation with tree-walker bridges installed.
    ///
    /// The bridges themselves are installed by [`install_vm_bridges`] — the
    /// single install site, shared with the bridged-parity tests.
    fn eval_with_flake_resolver(input: &str) -> Result<Value, EvalError> {
        let _bridges = install_vm_bridges();

        match sui_bytecode::eval_full(input) {
            Ok(result) => Ok(convert::string_keyed_to_eval(&result.to_string_keyed())),
            Err(sui_bytecode::EvalError::Compile(c)) => {
                // Compilation failed — fall back to tree-walker entirely.
                eprintln!("[sui-vm] top-level compile fallback: {c}");
                eval::eval(input)
            }
            Err(sui_bytecode::EvalError::Runtime(r)) => {
                // Runtime error — fall back to tree-walker entirely.
                // This handles VM bugs (GetLocal slot mismatch, etc.)
                // that don't affect correctness of the tree-walker.
                eprintln!("[sui-vm] top-level runtime fallback: {r}");
                eval::eval(input)
            }
        }
    }
}

/// Convert a tree-walker `Value` to a `StringKeyedValue` (no interner needed).
///
/// Used by the flake resolver bridge to convert tree-walker results
/// into a format the bytecode VM can consume.
///
/// **Lazy thunk handling:** Tree-walker thunks are NOT eagerly forced.
/// Instead, they are wrapped in `StringKeyedValue::Thunk` with a callback
/// that forces the underlying tree-walker thunk on demand. This is critical
/// for `getFlake` performance: a typical flake has 100+ transitive input
/// thunks, and forcing them all would trigger git clones, recursive flake
/// resolution, and full evaluation of every dependency (10s+). By wrapping
/// lazily, only the inputs actually accessed by the expression are evaluated.
pub fn eval_to_string_keyed(val: &Value) -> sui_bytecode::StringKeyedValue {
    match val {
        Value::Null => sui_bytecode::StringKeyedValue::Null,
        Value::Bool(b) => sui_bytecode::StringKeyedValue::Bool(*b),
        Value::Int(n) => sui_bytecode::StringKeyedValue::Int(*n),
        Value::Float(f) => sui_bytecode::StringKeyedValue::Float(*f),
        Value::String(s) => sui_bytecode::StringKeyedValue::String(s.chars.to_string()),
        Value::Path(p) => sui_bytecode::StringKeyedValue::Path(p.to_string()),
        Value::List(items) => {
            sui_bytecode::StringKeyedValue::List(
                items.iter().map(eval_to_string_keyed).collect(),
            )
        }
        Value::Attrs(attrs) => {
            let mut map = std::collections::BTreeMap::new();
            for (k, v) in attrs.iter() {
                map.insert(k.clone(), eval_to_string_keyed(v));
            }
            sui_bytecode::StringKeyedValue::Attrs(map)
        }
        Value::Lambda(closure) => {
            // Wrap in Rc so the Fn closure captures a shared pointer
            // rather than an owned Closure.  Each invocation clones out
            // of the Rc (all Closure fields are refcounted, so O(1)),
            // but the capture itself is just an Rc bump — no redundant
            // outer+inner double-clone.
            let closure_rc = std::rc::Rc::new((**closure).clone());
            sui_bytecode::StringKeyedValue::Callable(std::rc::Rc::new(move |arg| {
                let eval_arg = convert::string_keyed_to_eval(&arg);
                let func = Value::Lambda(Rc::new((*closure_rc).clone()));
                let result = eval::apply(func, eval_arg)
                    .map_err(|e| e.to_string())?;
                let forced = eval::force_value(&result)
                    .map_err(|e| e.to_string())?;
                Ok(eval_to_string_keyed(&forced))
            }))
        }
        Value::Builtin(bf) => {
            // Same Rc pattern as Lambda above — BuiltinFn::clone() is
            // already cheap (static str + Rc bump on func), but the Rc
            // wrapper avoids the misleading double-clone.
            let bf_rc = std::rc::Rc::new((**bf).clone());
            sui_bytecode::StringKeyedValue::Callable(std::rc::Rc::new(move |arg| {
                let eval_arg = convert::string_keyed_to_eval(&arg);
                let func = Value::Builtin(Box::new((*bf_rc).clone()));
                let result = eval::apply(func, eval_arg)
                    .map_err(|e| e.to_string())?;
                let forced = eval::force_value(&result)
                    .map_err(|e| e.to_string())?;
                Ok(eval_to_string_keyed(&forced))
            }))
        }
        Value::Thunk(t) => {
            // Fast path: if already evaluated, convert the memoized value
            // without creating a thunk wrapper.
            if t.is_evaluated() {
                match t.force(&|e, env| eval::eval_expr(e, env)) {
                    Ok(v) => eval_to_string_keyed(&v),
                    Err(_) => sui_bytecode::StringKeyedValue::Null,
                }
            } else {
                // LAZY: Wrap the tree-walker thunk in a callback that
                // forces it on demand. The Rc clone is cheap and keeps
                // the thunk's memoization cell shared, so forcing once
                // caches the result for all subsequent accesses.
                let thunk_clone = t.clone();
                sui_bytecode::StringKeyedValue::Thunk(std::rc::Rc::new(move || {
                    let forced = thunk_clone
                        .force(&|e, env| eval::eval_expr(e, env))
                        .map_err(|e| e.to_string())?;
                    Ok(eval_to_string_keyed(&forced))
                }))
            }
        }
    }
}

impl Evaluator for BytecodeEvaluator {
    fn eval_expr(&self, input: &str) -> Result<Value, EvalError> {
        Self::eval_with_flake_resolver(input)
    }

    fn eval_file(&self, path: &std::path::Path) -> Result<Value, EvalError> {
        let source = std::fs::read_to_string(path)
            .map_err(|e| EvalError::IoError {
                context: format!("eval_file: {}", path.display()),
                message: e.to_string(),
            })?;
        self.eval_expr(&source)
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    struct MockEvaluator(Result<Value, EvalError>);
    impl Evaluator for MockEvaluator {
        fn eval_expr(&self, _: &str) -> Result<Value, EvalError> {
            match &self.0 { Ok(v) => Ok(v.clone()), Err(_) => Err(EvalError::NotImplemented("mock".into())) }
        }
        fn eval_file(&self, _: &std::path::Path) -> Result<Value, EvalError> {
            self.eval_expr("")
        }
    }

    #[test]
    fn mock_evaluator_ok() {
        let e = MockEvaluator(Ok(Value::Int(42)));
        assert_eq!(e.eval_expr("anything").unwrap(), Value::Int(42));
    }

    #[test]
    fn mock_evaluator_err() {
        let e = MockEvaluator(Err(EvalError::NotImplemented("x".into())));
        assert!(e.eval_expr("anything").is_err());
    }

    #[test]
    fn tree_walk_evaluator() {
        let e = TreeWalkEvaluator;
        assert_eq!(e.eval_expr("1 + 2").unwrap(), Value::Int(3));
    }

    #[test]
    fn evaluator_trait_object_safe() {
        fn _assert(_: &dyn Evaluator) {}
    }

    // ── TreeWalkEvaluator through Evaluator trait ────────────

    #[test]
    fn tree_walk_eval_integer_arithmetic() {
        let e: &dyn Evaluator = &TreeWalkEvaluator;
        assert_eq!(e.eval_expr("2 + 3").unwrap(), Value::Int(5));
    }

    #[test]
    fn tree_walk_eval_string_literal() {
        let e: &dyn Evaluator = &TreeWalkEvaluator;
        assert_eq!(
            e.eval_expr(r#""hello world""#).unwrap(),
            Value::string("hello world"),
        );
    }

    #[test]
    fn tree_walk_eval_boolean() {
        let e: &dyn Evaluator = &TreeWalkEvaluator;
        assert_eq!(e.eval_expr("true && false").unwrap(), Value::Bool(false));
    }

    #[test]
    fn tree_walk_eval_if_else() {
        let e: &dyn Evaluator = &TreeWalkEvaluator;
        assert_eq!(
            e.eval_expr("if true then 42 else 0").unwrap(),
            Value::Int(42),
        );
    }

    #[test]
    fn tree_walk_eval_let_binding() {
        let e: &dyn Evaluator = &TreeWalkEvaluator;
        assert_eq!(
            e.eval_expr("let x = 10; in x * 2").unwrap(),
            Value::Int(20),
        );
    }

    #[test]
    fn tree_walk_eval_attrset() {
        let e: &dyn Evaluator = &TreeWalkEvaluator;
        let val = e.eval_expr("{ a = 1; b = 2; }.a").unwrap();
        assert_eq!(val, Value::Int(1));
    }

    #[test]
    fn tree_walk_eval_list() {
        let e: &dyn Evaluator = &TreeWalkEvaluator;
        let val = e.eval_expr("[1 2 3]").unwrap();
        assert_eq!(
            val,
            Value::list(vec![Value::Int(1), Value::Int(2), Value::Int(3)]),
        );
    }

    #[test]
    fn tree_walk_eval_lambda_application() {
        let e: &dyn Evaluator = &TreeWalkEvaluator;
        assert_eq!(
            e.eval_expr("(x: x + 1) 5").unwrap(),
            Value::Int(6),
        );
    }

    #[test]
    fn tree_walk_eval_builtin_via_trait() {
        let e: &dyn Evaluator = &TreeWalkEvaluator;
        assert_eq!(
            e.eval_expr("builtins.length [1 2 3]").unwrap(),
            Value::Int(3),
        );
    }

    #[test]
    fn tree_walk_eval_parse_error_via_trait() {
        let e: &dyn Evaluator = &TreeWalkEvaluator;
        let result = e.eval_expr("let in");
        assert!(result.is_err());
    }

    #[test]
    fn tree_walk_eval_null_via_trait() {
        let e: &dyn Evaluator = &TreeWalkEvaluator;
        assert_eq!(e.eval_expr("null").unwrap(), Value::Null);
    }

    #[test]
    fn tree_walk_eval_file_missing() {
        let e: &dyn Evaluator = &TreeWalkEvaluator;
        let result = e.eval_file(std::path::Path::new("/nonexistent/file.nix"));
        assert!(result.is_err());
    }

    #[test]
    fn tree_walk_eval_string_interpolation_via_trait() {
        let e: &dyn Evaluator = &TreeWalkEvaluator;
        assert_eq!(
            e.eval_expr(r#"let name = "world"; in "hello ${name}""#).unwrap(),
            Value::string("hello world"),
        );
    }

    #[test]
    fn tree_walk_eval_comparison_via_trait() {
        let e: &dyn Evaluator = &TreeWalkEvaluator;
        assert_eq!(e.eval_expr("3 > 2").unwrap(), Value::Bool(true));
        assert_eq!(e.eval_expr("1 == 1").unwrap(), Value::Bool(true));
    }

    #[test]
    fn tree_walk_eval_recursive_attrset_via_trait() {
        let e: &dyn Evaluator = &TreeWalkEvaluator;
        assert_eq!(
            e.eval_expr("rec { x = 1; y = x + 1; }.y").unwrap(),
            Value::Int(2),
        );
    }

    // ── Re-exports & convenience eval shim ─────────────────

    #[test]
    fn re_export_eval_function_works() {
        // The top-level `eval` re-export from `eval::eval` should be
        // identical to the inner function.
        assert_eq!(eval("1 + 1").unwrap(), Value::Int(2));
    }

    #[test]
    fn re_export_value_and_error_types_constructible() {
        let v: Value = Value::Int(7);
        let e: EvalError = EvalError::UndefinedVar("x".into());
        assert_eq!(v.type_name(), "int");
        assert!(e.to_string().contains("undefined"));
    }

    // ── flake re-export from sui-compat ────────────────────

    #[test]
    fn flake_module_re_exports_compat_types() {
        // Smoke test that the flake submodule re-export compiles. We
        // reference a known type from the sui_compat::flake module via
        // our own re-export. Whatever types live there must be reachable.
        // We use the path explicitly to force compilation of the import.
        #[allow(unused_imports)]
        use crate::flake::*;
        // The block is intentionally empty: success is "this compiled".
    }

    // ── TreeWalkEvaluator passes path through ──────────────

    #[test]
    fn tree_walk_eval_file_with_real_temp_file() {
        let dir = std::env::temp_dir().join("sui-eval-test-tree-walk");
        let _ = std::fs::create_dir_all(&dir);
        let path = dir.join("simple.nix");
        std::fs::write(&path, "1 + 2").unwrap();
        let e: &dyn Evaluator = &TreeWalkEvaluator;
        let result = e.eval_file(&path).unwrap();
        assert_eq!(result, Value::Int(3));
        let _ = std::fs::remove_file(&path);
        let _ = std::fs::remove_dir(&dir);
    }

    #[test]
    fn tree_walk_eval_file_propagates_io_error_kind() {
        let e: &dyn Evaluator = &TreeWalkEvaluator;
        let result = e.eval_file(std::path::Path::new("/nonexistent/never/exists.nix"));
        match result {
            Err(EvalError::IoError { context, .. }) => {
                assert!(context.contains("eval_file"));
            }
            other => panic!("expected IoError, got {other:?}"),
        }
    }

    #[test]
    fn tree_walk_eval_file_parse_error_propagates() {
        let dir = std::env::temp_dir().join("sui-eval-test-tw-parse");
        let _ = std::fs::create_dir_all(&dir);
        let path = dir.join("bad.nix");
        std::fs::write(&path, "let in").unwrap();
        let e: &dyn Evaluator = &TreeWalkEvaluator;
        let result = e.eval_file(&path);
        assert!(result.is_err());
        let _ = std::fs::remove_file(&path);
        let _ = std::fs::remove_dir(&dir);
    }

    // ── Mock evaluator additional ──────────────────────────

    #[test]
    fn mock_evaluator_dispatched_via_trait_object() {
        let m: Box<dyn Evaluator> = Box::new(MockEvaluator(Ok(Value::Bool(true))));
        let r = m.eval_expr("anything").unwrap();
        assert_eq!(r, Value::Bool(true));
    }

    #[test]
    fn mock_evaluator_eval_file_routes_through_eval_expr() {
        let m = MockEvaluator(Ok(Value::Int(1)));
        let r = m.eval_file(std::path::Path::new("/dev/null"));
        assert_eq!(r.unwrap(), Value::Int(1));
    }

    // ── Through-trait coverage of more constructs ──────────

    #[test]
    fn tree_walk_eval_function_with_default_args() {
        let e: &dyn Evaluator = &TreeWalkEvaluator;
        assert_eq!(
            e.eval_expr("({a, b ? 10}: a + b) {a = 5;}").unwrap(),
            Value::Int(15),
        );
    }

    #[test]
    fn tree_walk_eval_with_throws_propagated() {
        let e: &dyn Evaluator = &TreeWalkEvaluator;
        let result = e.eval_expr(r#"builtins.throw "boom""#);
        assert!(result.is_err());
        let err = result.unwrap_err();
        assert!(err.is_throw());
    }

    #[test]
    fn tree_walk_eval_assert_failure() {
        let e: &dyn Evaluator = &TreeWalkEvaluator;
        let result = e.eval_expr("assert false; 42");
        assert!(matches!(result, Err(EvalError::AssertionFailed(_))));
    }

    #[test]
    fn tree_walk_eval_division_by_zero() {
        let e: &dyn Evaluator = &TreeWalkEvaluator;
        let result = e.eval_expr("1 / 0");
        assert!(matches!(result, Err(EvalError::DivisionByZero)));
    }

    #[test]
    fn tree_walk_eval_undefined_variable() {
        let e: &dyn Evaluator = &TreeWalkEvaluator;
        let result = e.eval_expr("nonexistent_xyz");
        assert!(matches!(result, Err(EvalError::UndefinedVar(_))));
    }

    #[test]
    fn tree_walk_eval_path_literal() {
        let e: &dyn Evaluator = &TreeWalkEvaluator;
        let v = e.eval_expr("/tmp/x").unwrap();
        assert!(matches!(v, Value::Path(_)));
    }

    #[test]
    fn tree_walk_eval_float_literal() {
        let e: &dyn Evaluator = &TreeWalkEvaluator;
        assert_eq!(e.eval_expr("3.14").unwrap(), Value::Float(3.14));
    }

    #[test]
    fn tree_walk_eval_lambda_returns_lambda() {
        let e: &dyn Evaluator = &TreeWalkEvaluator;
        let v = e.eval_expr("x: x").unwrap();
        assert!(matches!(v, Value::Lambda(_)));
    }
}