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
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
|
package examples.tcpoly.monad;
trait Monads {
/**
* class Monad m where
* (>>=) :: m a -> (a -> m b) -> m b
* return :: a -> m a
*
* MonadTC encodes the above Haskell type class,
* an instance of MonadTC corresponds to a method dictionary.
* (see http://lampwww.epfl.ch/~odersky/talks/wg2.8-boston06.pdf)
*
* Note that the identity (`this') of the method dictionary does not really correspond
* to the instance of m[x] (`self') that is `wrapped': e.g., unit does not use `self' (which
* corresponds to the argument of the implicit conversion that encodes an instance of this type class)
*/
// Option =:= [x] => Option[x] <: [x] => Any
// trait MonadTC[m <: [x] => Any, a] {
// MonadTC[m[x], a] x is a type parameter too -- should not write e.g., m[Int] here
trait MonadTC[m[x], a] {
def unit[a](orig: a): m[a]
// >>='s first argument comes from the implicit definition constructing this "method dictionary"
def >>=[b](fun: a => m[b]): m[b]
}
}
/**
* instance Monad Maybe where
* (Just x) >>= k = k x
* Nothing >>= _ = Nothing
*/
trait OptionMonad extends Monads {
// this implicit method encodes the Monad type class instance for Option
implicit def OptionInstOfMonad[a](self: Option[a]): MonadTC[Option, a]
= new MonadTC[Option, a] {
def unit[a](orig: a) = Some(orig)
def >>=[b](fun: a => Option[b]): Option[b] = self match {
case Some(x) => fun(x)
case None => None
}
}
}
object main extends OptionMonad with Application {
Console.println(Some("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa") >>= (x => Some(x.length)))
}
/*
trait MonadTC[m[x], a] requires m[x] {
def unit[a](orig: a): m[a]
// >>='s first argument comes from the implicit definition constructing this "method dictionary"
def >>=[b](fun: a => m[b]): m[b]
}
abstract class OptionIsMonad[t[x] <: Option[x], a] implicit extends MonadTC[t, a] {
def unit[a](orig: a) = Some(orig) // TODO: problematic.. is a meta-member: not invoked on this
}
class SomeIsMonad[a] extends OptionIsMonad[Some, a] {
def >>=[b](fun: a => Option[b]): Option[b] = fun(x)
}
class NoneIsMonad[a] extends OptionIsMonad[None, a] {
def >>=[b](fun: a => Option[b]): Option[b] = None
}
*/
|
