#include "share/atspre_staload.hats"
datatype maybe (a:t@ype) =
| Nothing (a) of ()
| Just (a) of a
sortdef _tycon = t@ype -> type
extern fun {m:_tycon} {a:t@ype} monad$pure (a): m(a)
extern fun {m:_tycon} {a,b:t@ype} monad$bind (m(a), a -<cloref1> m(b)): m(b)
implement (a:t@ype) monad$pure<maybe><a> (x) =
Just (x)
implement (a,b:t@ype) monad$bind<maybe><a,b> (x, f) =
case+ x of
| Just x => f x
| Nothing () => Nothing ()
infixl >>=
macdef >>= (x, f) = monad$bind (,(x), ,(f))
implement main0 () = let
val x = monad$pure<maybe><int> 10
val n = x >>= (lam x => Just (x+32))
>>= (lam y => let val _ = print_int y in Just " is the answer." end)
>>= (lam z => let val _ = print_string z in Nothing {int} () end)
: maybe int
in
end
