do_notation/

lib.rs

//! # `do-notation`, the monadic `do` notation brought to Rust.
//!
//! This crate provides the `m!` macro, which provides the Haskell monadic syntactic sugar `do`.
//!
//! > Note: it is not possible to use the `do!` syntax as `do` is a reserved keyword in Rust.
//!
//! The syntax is very similar to what you find in Haskell:
//!
//! - You use the `m!` macro; in Haskell, you use the `do` keyword.
//! - The `<-` syntactic sugar binds its left hand side to the monadic right hand side
//!   by _entering_ the right side via a closure.
//! - Like almost any statement in Rust, you must end your statement with a semicolon (`;`).
//! - The last line must be absent of `;` or contains the `return` keyword.
//! - You can use `return` nowhere but on the last line.
//! - A line containing a single expression with a semicolon is a valid statement and has the same effect as `_ <- expr`.
//! - `let` bindings are allowed in the form `let <pattern> = <expr>;` and have the regular Rust meaning.
//!
//! ## How do I make my monad works with `m!`?
//!
//! Because monads are higher-kinded types, it is not possible to define the monadic do-notation in a fully type-system
//! elegant way. However, this crate is based on the rebindable concept in Haskell (i.e. you can change what the `>>=`
//! operator’s types are), so `m!` has one type-system requirement and one syntactic requirement.
//!
//! First, you have to implement one trait: [`Lift`], which allows to _lift_ a value `A` into a _monadic structure of
//! `A`_. For instance, lifting a `A` into the `Option` monad yields an `Option<A>`.
//!
//! Then, you have to provide an `and_then` method, which is akin to Haskell’s `>>=` operator. The choice of using
//! `and_then` and not a proper name like `flat_map` or `bind` is due to the current state of the standard-library —
//! monads like `Option` and `Result<_, E>` don’t have `flat_map` defined on them but have `and_then`. The type signature
//! is not enforced, but:
//!
//! - `and_then` must be a binary function taking a type `A`, a closure `A -> Monad<B>` and returns `Monad<B>`, where
//!   `Monad` is the monad you are adding `and_then` for. For instance, if you are implementing it for `Option`,
//!   `and_then` takes an `A`, a closure `A -> Option<B>` and returns an `Option<B>`.
//! - `and_then` must move its first argument, which has to be `self`. The type of `Self` is not enforced.
//! - `and_then`’s closure must take `A` with a `FnOnce` closure.
//!
//! ## Meaning of the `<-` operator
//!
//! The `<-` syntactic sugar is not strictly speaking an operator: it’s not valid vanilla Rust. Instead, it’s a trick
//! defined in the `m!` allowing to use both [`Lift::lift`] and `and_then`. When you look at code inside a do-notation
//! block, every monadic statements (separated with `;` in this crate) can be imagined as a new level of nesting inside
//! a closure — the one passed to `and_then`, indeed.
//!
//! ## First example: fallible code
//!
//! One of the first monadic application that people learn is the _fallible_ effect — `Maybe` in Haskell.
//! In `Rust`, it’s `Option`. `Option` is an interesting monad as it allows you to fail early.
//!
//! ```rust
//! use do_notation::m;
//!
//! let r = m! {
//!   x <- Some("Hello, world!");
//!   y <- Some(3);
//!   Some(x.len() * y)
//! };
//!
//! assert_eq!(r, Some(39));
//! ```
//!
//! The `binding <- expr` syntax unwraps the right part and binds it to `binding`, making it available to
//! next calls — remember, nested closures. The final line re-enters the structure (here, `Option`) explicitly.
//!
//! Note that it is possible to re-enter the structure without having to specify how / knowing the structure
//! (with `Option`, you re-enter with `Some`). You can use the `return` keyword, that will automatically lift the
//! value into the right structure:
//!
//! ```rust
//! use do_notation::m;
//!
//! let r = m! {
//!   x <- Some(1);
//!   y <- Some(2);
//!   z <- Some(3);
//!   return [x, y, z];
//! };
//!
//! assert_eq!(r, Some([1, 2, 3]));
//! ```

#[macro_export]
macro_rules! m {
  // return
  (return $r:expr ;) => {
    $crate::Lift::lift($r)
  };

  // let-binding
  (let $p:pat = $e:expr ; $($r:tt)*) => {{
    let $p = $e;
    m!($($r)*)
  }};

  // const-bind
  (_ <- $x:expr ; $($r:tt)*) => {
    $x.and_then(move |_| { m!($($r)*) })
  };

  // bind
  ($binding:ident <- $x:expr ; $($r:tt)*) => {
    $x.and_then(move |$binding| { m!($($r)*) })
  };

  // const-bind
  ($e:expr ; $($a:tt)*) => {
    $e.and_then(move |_| m!($($a)*))
  };

  // pure
  ($a:expr) => {
    $a
  }
}

/// Lift a value inside a monad.
pub trait Lift<A> {
  /// Lift a value into a default structure.
  fn lift(a: A) -> Self;
}

impl<A> Lift<A> for Option<A> {
  fn lift(a: A) -> Self {
    Some(a)
  }
}

impl<A, E> Lift<A> for Result<A, E> {
  fn lift(a: A) -> Self {
    Ok(a)
  }
}

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

  #[test]
  fn option() {
    let r: Option<i32> = m! {
      v <- Some(3);
      Some(v)
    };

    assert_eq!(r, Some(3));

    let r: Option<i32> = m! {
      v <- r;
      x <- Some(10);
      Some(v * x)
    };

    assert_eq!(r, Some(30));

    let n: Option<i32> = None;
    let r: Option<i32> = m! {
      v <- Some(314);
      x <- n;
      Some(v * x)
    };

    assert_eq!(r, None);

    let r = m! {
      _ <- Some("a");
      b <- Some("b");
      _ <- Option::<&str>::None;
      Some(b)
    };

    assert_eq!(r, None);

    let r = m! {
      _ <- Some("a");
      return "b";
    };

    assert_eq!(r, Some("b"));
  }

  #[test]
  fn result() {
    let r: Result<i32, &str> = m! {
      v <- Ok(3);
      Ok(v)
    };

    assert_eq!(r, Ok(3));

    let r: Result<i32, &str> = m! {
      v <- r;
      x <- Ok(10);
      Ok(v * x)
    };

    assert_eq!(r, Ok(30));

    let n: Result<i32, &str> = Err("error");
    let r: Result<i32, &str> = m! {
      v <- Ok(314);
      x <- n;
      Ok(v * x)
    };

    assert_eq!(r, Err("error"));

    let r = m! {
      _ <- Result::<&str, &str>::Ok("a");
      b <- Ok("b");
      _ <- Result::<&str, &str>::Err("nope");
      Ok(b)
    };

    assert_eq!(r, Err("nope"));

    fn guard<E>(cond: bool, err: E) -> Result<(), E> {
      if cond {
        Ok(())
      } else {
        Err(err)
      }
    }

    let r = m! {
      x <- Ok(true);
      _ <- guard(1 == 2, "meh");
      Ok(x)
    };

    assert_eq!(r, Err("meh"));
  }

  #[test]
  fn instruction_counter() {
    struct IC<A> {
      count: usize,
      value: A,
    }

    impl<A> IC<A> {
      fn new(value: A) -> Self {
        IC { count: 1, value }
      }

      fn value(&self) -> &A {
        &self.value
      }

      fn count(&self) -> usize {
        self.count
      }

      fn and_then<B>(self, f: impl FnOnce(A) -> IC<B>) -> IC<B> {
        let r = f(self.value);

        IC {
          count: self.count + r.count,
          value: r.value,
        }
      }
    }

    impl<A> Lift<A> for IC<A> {
      fn lift(value: A) -> Self {
        Self::new(value)
      }
    }

    let ic = m! {
      a <- IC::new(10);
      b <- IC::new(2);
      IC::new(a + b)
    };

    assert_eq!(ic.value(), &12);
    assert_eq!(ic.count(), 3);

    let ic = m! {
      _ <- IC::new("a");
      let x = 2;

      // test statements
      let y = if 1 == 1 { 3 } else { 0 };

      _ <- IC::new("b");
      _ <- IC::new("c");

      return [1, x, y];
    };

    assert_eq!(ic.value(), &[1, 2, 3]);
    assert_eq!(ic.count(), 4);
  }
}