src/test/ui/monad.rs

   1 // run-pass
   2
   3 #![allow(non_camel_case_types)]
   4
   5
   6
   7 trait vec_monad<A> {
   8     fn bind<B, F>(&self, f: F ) -> Vec<B> where F: FnMut(&A) -> Vec<B> ;
   9 }
  10
  11 impl<A> vec_monad<A> for Vec<A> {
  12     fn bind<B, F>(&self, mut f: F) -> Vec<B> where F: FnMut(&A) -> Vec<B> {
  13         let mut r = Vec::new();
  14         for elt in self {
  15             r.extend(f(elt));
  16         }
  17         r
  18     }
  19 }
  20
  21 trait option_monad<A> {
  22     fn bind<B, F>(&self, f: F) -> Option<B> where F: FnOnce(&A) -> Option<B>;
  23 }
  24
  25 impl<A> option_monad<A> for Option<A> {
  26     fn bind<B, F>(&self, f: F) -> Option<B> where F: FnOnce(&A) -> Option<B> {
  27         match *self {
  28             Some(ref a) => { f(a) }
  29             None => { None }
  30         }
  31     }
  32 }
  33
  34 fn transform(x: Option<isize>) -> Option<String> {
  35     x.bind(|n| Some(*n + 1) ).bind(|n| Some(n.to_string()) )
  36 }
  37
  38 pub fn main() {
  39     assert_eq!(transform(Some(10)), Some("11".to_string()));
  40     assert_eq!(transform(None), None);
  41     assert_eq!((vec!["hi".to_string()])
  42         .bind(|x| vec![x.clone(), format!("{}!", x)] )
  43         .bind(|x| vec![x.clone(), format!("{}?", x)] ),
  44         ["hi".to_string(),
  45          "hi?".to_string(),
  46          "hi!".to_string(),
  47          "hi!?".to_string()]);
  48 }