Monads/Maybe monad: Difference between revisions
→{{header|C}}
Line 305:
{missing value, missing value, missing value, missing value, 0.5, 0.0, -0.346573590279, -0.499999999999, -0.549306144333, -0.69314718056, -0.804718956217}</pre>
=={{header|ATS}}==
<syntaxhighlight lang="ats">
#include "share/atspre_staload.hats"
(* There are "Option" and "Option_vt" in the ATS2 prelude, but I shall
construct something anew. *)
datatype Maybe (a : t@ype+) =
| Nothing of ()
| Just of a
fn {a, b : t@ype}
bind_Maybe {u : bool}
(m : Maybe a,
f : a -<cloref1> Maybe b) : Maybe b =
case+ m of
| Nothing {a} () => Nothing {b} ()
| Just {a} x => f x
infixl 0 >>=
overload >>= with bind_Maybe
implement
main0 () =
let
val f : int -<cloref1> Maybe int =
lam i =<cloref1> if (i : int) < 0 then Nothing () else Just i
val g : int -<cloref1> Maybe string =
lam i =<cloref1> Just (tostring_val<int> i)
in
case+ Just 123 >>= f >>= g of
| Nothing () => println! ("Nothing ()")
| Just s => println! ("Just (\"", s : string, "\")");
case+ Just ~123 >>= f >>= g of
| Nothing () => println! ("Nothing ()")
| Just s => println! ("Just (\"", s : string, "\")")
end
</syntaxhighlight>
{{out}}
<pre>$ patscc -std=gnu2x -g -O2 -DATS_MEMALLOC_GCBDW maybe_monad_ats.dats -lgc && ./a.out
Just ("123")
Nothing ()</pre>
=={{header|C}}==
| |||