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