src/test/run-pass/monad.rs

   1 // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11
  12
  13
  14 trait vec_monad<A> {
  15     fn bind<B, F>(&self, f: F ) -> Vec<B> where F: FnMut(&A) -> Vec<B> ;
  16 }
  17
  18 impl<A> vec_monad<A> for Vec<A> {
  19     fn bind<B, F>(&self, mut f: F) -> Vec<B> where F: FnMut(&A) -> Vec<B> {
  20         let mut r = Vec::new();
  21         for elt in self {
  22             r.extend(f(elt));
  23         }
  24         r
  25     }
  26 }
  27
  28 trait option_monad<A> {
  29     fn bind<B, F>(&self, f: F) -> Option<B> where F: FnOnce(&A) -> Option<B>;
  30 }
  31
  32 impl<A> option_monad<A> for Option<A> {
  33     fn bind<B, F>(&self, f: F) -> Option<B> where F: FnOnce(&A) -> Option<B> {
  34         match *self {
  35             Some(ref a) => { f(a) }
  36             None => { None }
  37         }
  38     }
  39 }
  40
  41 fn transform(x: Option<isize>) -> Option<String> {
  42     x.bind(|n| Some(*n + 1) ).bind(|n| Some(n.to_string()) )
  43 }
  44
  45 pub fn main() {
  46     assert_eq!(transform(Some(10)), Some("11".to_string()));
  47     assert_eq!(transform(None), None);
  48     assert_eq!((vec!("hi".to_string()))
  49         .bind(|x| vec!(x.clone(), format!("{}!", x)) )
  50         .bind(|x| vec!(x.clone(), format!("{}?", x)) ),
  51         ["hi".to_string(),
  52          "hi?".to_string(),
  53          "hi!".to_string(),
  54          "hi!?".to_string()]);
  55 }