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(* ****** ****** *)
//
// Experimenting with maybe-monad
//
(* ****** ****** *)
(*
** Author: Hongwei Xi
** Authoremail: hwxi AT cs DOT bu DOT edu
** Time: the third of July, 2013
*)
(* ****** ****** *)
//
#include
"share/atspre_staload.hats"
//
(* ****** ****** *)
abstype monad_type (a: t@ype) = ptr
typedef monad (a: t@ype) = monad_type (a)
(* ****** ****** *)
typedef
cfun1 (a: t@ype, b: t@ype) = a -<cloref1> b
typedef
cfun2 (a1: t@ype, a2: t@ype, b: t@ype) = (a1, a2) -<cloref1> b
(* ****** ****** *)
stadef cfun = cfun1
stadef cfun = cfun2
(* ****** ****** *)
extern
fun{
a,b:t@ype
} monad_bind
(monad (a), cfun (a, monad (b))): monad (b)
// end of [monad_bind]
extern
fun{a:t@ype} monad_return (x: a): monad (a)
extern
fun{a:t@ype} monad_unretn (m: monad (a)): Option (a)
(* ****** ****** *)
extern
fun{
a,b:t@ype
} monad_fmap (f: cfun (a, b), m: monad (a)): monad (b)
extern
fun{
a,b:t@ype
} monad_lift (f: cfun (a, b), m: monad (a)): monad (b)
(* ****** ****** *)
implement
{a,b}
monad_fmap (f, m) =
monad_bind<a,b> (m, lam (x) => monad_return<b> (f (x)))
// end of [monad_fmap]
implement
{a,b}
monad_lift (f, m) = monad_fmap<a,b> (f, m)
(* ****** ****** *)
local
assume monad_type (a: t@ype) = Option a
in (* in of [local] *)
implement{a,b}
monad_bind (m, f) =
(
case m of Some (x) => f (x) | None () => None ()
) // end of [monad_bind]
implement{a}
monad_return (x) = Some{a}(x)
implement{a}
monad_unretn (m) = m
end // end of [local]
(* ****** ****** *)
#define mbind monad_bind
#define mretn monad_return
#define mlift monad_lift
#define mfmap monad_fmap
#define munretn monad_unretn
(* ****** ****** *)
fun
test_maybe
(
x0: int
) : int = let
//
val m0 = mretn<int> (x0)
val m0 = mlift<int,int> (lam x => x+3, m0)
val m0 = mbind<int,int> (m0, lam x => mretn (x*2))
val-Some(res) = munretn<int> (m0)
//
in
res (* = (x0+3)*2 *)
end // end of [test_maybe]
(* ****** ****** *)
val () = assertloc (test_maybe(10) = (10+3)*2)
(* ****** ****** *)
implement main0 () = ()
(* ****** ****** *)
(* end of [monad_maybe.dats] *)
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