mdo/lib.rs
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// Copyright (c) 2014 Guillaume Pinot <texitoi(a)texitoi.eu>
//
// This work is free. You can redistribute it and/or modify it under
// the terms of the Do What The Fuck You Want To Public License,
// Version 2, as published by Sam Hocevar. See the COPYING file for
// more details.
#![deny(missing_docs)]
#![deny(warnings)]
//! Monadic do notation
/// Monadic do notation using duck typing
///
/// Syntax:
/// `(instr)* ; ret expr`
///
/// instr can be:
///
/// * `pattern =<< expression`: bind expression to pattern. a `bind`
/// function must be in scope.
///
/// * `let pattern = expression`: assign expression to pattern, as
/// normal rust let.
///
/// * `ign expression`: equivalent to `_ =<< expression`
///
/// * `when expression`: filter on the monad. `ret` and `mzero`
/// functions must be in scope.
///
/// # Example
///
/// ```
/// #[macro_use] extern crate mdo;
/// fn main() {
/// use mdo::iter::{bind, ret, mzero};
/// let l = mdo! {
/// x =<< 0i32..5; // assign x to [0, 5[
/// ign 0..2; // duplicate each value
/// when x % 2 == 0; // filter on even values
/// let y = x + 5; // create y
/// ret ret(y + 5) // return y + 5
/// }.collect::<Vec<_>>();
/// assert_eq!(l, vec![10, 10, 12, 12, 14, 14]);
/// }
/// ```
#[macro_export]
macro_rules! mdo {
(
let $p: pat = $e: expr ; $( $t: tt )*
) => (
{ let $p = $e ; mdo! { $( $t )* } }
);
(
let $p: ident : $ty: ty = $e: expr ; $( $t: tt )*
) => (
{ let $p: $ty = $e ; mdo! { $( $t )* } }
);
(
$p: pat =<< $e: expr ; $( $t: tt )*
) => (
bind($e, move |$p| mdo! { $( $t )* } )
);
(
$p: ident : $ty: ty =<< $e: expr ; $( $t: tt )*
) => (
bind($e, move |$p : $ty| mdo! { $( $t )* } )
);
(
ign $e: expr ; $( $t: tt )*
) => (
bind($e, move |_| mdo! { $( $t )* })
);
(
when $e: expr ; $( $t: tt )*
) => (
bind(if $e { ret(()) } else { mzero() }, move |_| mdo! { $( $t )* })
);
(
ret $f: expr
) => (
$f
)
}
pub mod option {
//! Monadic functions for Option<T>
/// bind for Option<T>, equivalent to `m.and_then(f)`
pub fn bind<T, U, F: FnOnce(T) -> Option<U>>(m: Option<T>, f: F) -> Option<U> {
m.and_then(f)
}
/// return for Option<T>, equivalent to `Some(x)`
pub fn ret<T>(x: T) -> Option<T> {
Some(x)
}
/// mzero for Option<T>, equivalent to `None`
pub fn mzero<T>() -> Option<T> {
None
}
}
pub mod result {
//! Monadic functions for Result<T, E>
/// bind for Result<T, E>, equivalent to `m.and_then(f)`
pub fn bind<T, E, U, F: FnOnce(T) -> Result<U, E>>(m: Result<T, E>, f: F) -> Result<U, E> {
m.and_then(f)
}
/// return for Result<T, E>, equivalent to `Ok(x)`
pub fn ret<T, E>(x: T) -> Result<T, E> {
Ok(x)
}
}
pub mod iter {
//! Monadic functions for Iterator<T>
use std::option;
use std::iter::FlatMap;
/// bind for Iterator<T, E>, equivalent to `m.flat_map(f)`
pub fn bind<I, U, F>(m: I, f: F) -> FlatMap<I, U, F>
where I: Iterator, U: Iterator, F: FnMut(<I as Iterator>::Item) -> U {
m.flat_map(f)
}
/// return for Iterator<T>, an iterator with one value.
pub fn ret<T>(x: T) -> option::IntoIter<T> {
Some(x).into_iter()
}
/// mzero for Iterator<T>, an empty iterator.
pub fn mzero<T>() -> option::IntoIter<T> {
None.into_iter()
}
}
#[cfg(test)]
mod tests {
#[test]
fn option_bind() {
use super::option::{bind, ret, mzero};
let x = ret(5);
assert_eq!(x, Some(5));
let x = bind(ret(5), |x| ret(x + 1));
assert_eq!(x, Some(6));
let x = bind(ret(5), |x| bind(ret(x + 5), |x| ret(x * 2)));
assert_eq!(x, Some(20));
let x = bind(ret(5i32), |x| bind(if x == 0 { ret(()) } else { mzero() },
|_| ret(x * 2)));
assert_eq!(x, None);
}
#[test]
fn option_mdo() {
use super::option::{bind, ret, mzero};
let x = mdo! {
ret ret(5)
};
assert_eq!(x, Some(5));
let x = mdo! {
x =<< ret(5);
ret ret(x + 1)
};
assert_eq!(x, Some(6));
let x = mdo! {
x =<< ret(5);
x =<< ret(x + 5);
ret ret(x * 2)
};
assert_eq!(x, Some(20));
let x = mdo! {
x =<< ret(5i32);
when x == 0;
ret ret(x * 2)
};
assert_eq!(x, None);
}
#[test]
fn let_type() {
let _: i32 = mdo! {
let i: i32 = 0;
ret i
};
}
#[test]
fn iter_bind() {
use super::iter::{bind, ret, mzero};
let l = bind(0..3, move |x| x..3);
assert_eq!(l.collect::<Vec<_>>(), vec![0, 1, 2, 1, 2, 2]);
let l = bind(0i32..3, move |x|
bind(0..3, move |y| ret(x + y)));
assert_eq!(l.collect::<Vec<_>>(), vec![0, 1, 2, 1, 2, 3, 2, 3, 4]);
let l = bind(1i32..11, move |z|
bind(1..z + 1, move |y|
bind(1..y + 1, move |x|
bind(if x * x + y * y == z * z { ret(()) }
else { mzero() },
move |_|
ret((x, y, z))))));
assert_eq!(l.collect::<Vec<_>>(), vec![(3, 4, 5), (6, 8, 10)]);
}
#[test]
fn iter_mdo() {
use super::iter::{bind, ret, mzero};
let l = mdo! {
x =<< 0..3;
ret x..3
}.collect::<Vec<_>>();
assert_eq!(l, vec![0, 1, 2, 1, 2, 2]);
let l = mdo! {
x =<< 0i32..3;
y =<< 0..3;
ret ret(x + y)
}.collect::<Vec<_>>();
assert_eq!(l, vec![0, 1, 2, 1, 2, 3, 2, 3, 4]);
let l = mdo! {
z =<< 1i32..11;
y =<< 1..z;
x =<< 1..y + 1;
let test = x * x + y * y == z * z;
when test;
let res = (x, y, z);
ret ret(res)
}.collect::<Vec<_>>();
assert_eq!(l, vec![(3, 4, 5), (6, 8, 10)]);
}
#[test]
fn iter_ignore() {
use super::iter::{bind, ret};
let l = mdo! {
x =<< 0i32..5;
ign 0..2;
ret ret(x)
}.collect::<Vec<_>>();
assert_eq!(l, vec![0, 0, 1, 1, 2, 2, 3, 3, 4, 4]);
}
#[test]
fn ret_trick() {
use super::iter::bind;
let l = mdo! {
ret =<< 0..5;
ret 0..ret
}.collect::<Vec<_>>();
assert_eq!(l, vec![0, 0, 1, 0, 1, 2, 0, 1, 2, 3]);
}
#[test]
fn when_trick() {
use super::iter::{bind, ret, mzero};
let l = mdo! {
when =<< 0i32..5;
when when != 3;
ret ret(when)
}.collect::<Vec<_>>();
assert_eq!(l, vec![0, 1, 2, 4]);
}
#[test]
fn ign_trick() {
use super::iter::{bind, ret};
let l = mdo! {
ign =<< 0i32..5;
ign 0..0;
ret ret(ign)
}.collect::<Vec<_>>();
assert_eq!(l, vec![]);
}
#[test]
fn mdo_doc_example() {
use super::iter::{bind, ret, mzero};
let l = mdo! {
x: i32 =<< 0..5; // assign x to [0, 5[
ign 0..2; // duplicate each value
when x % 2 == 0; // filter on even values
let y = x + 5; // create y
ret ret(y + 5) // return y + 5
}.collect::<Vec<_>>();
assert_eq!(l, vec![10, 10, 12, 12, 14, 14]);
}
}