examples/demo/maybe.spl

   1 //poor mans maybe
   2 //(1, a) = Just a
   3 //(0, _) = Nothing
   4
   5 pure(a) :: a -> (Int, a) {
   6     return (1, a);
   7 }
   8
   9 fmap(f, ma) :: (a -> b) -> (Int, a) -> (Int, b) {
  10     var t = ma.snd; //copy to avoid editing ma by reference
  11     if (ma.fst == 0) { return ma; }
  12     else {
  13         t = f(ma.snd);
  14         return (ma.fst, t);
  15     }
  16 }
  17
  18
  19 bind(ma, f) :: (Int, a) -> (a -> (Int, b)) -> (Int, b) {
  20     if (ma.fst == 0) { return ma; }
  21     else {
  22         return f(ma.snd);
  23     }
  24 }
  25
  26 main() {
  27
  28     var j = pure(True);
  29     var n = (0, False); //ugly that we need the additional parameter
  30
  31     var jf = fmap(\b->!b, j);
  32     var nf = fmap(\b->!b, n);
  33
  34     var fma = \b -> pure(!b);
  35
  36     var bjf = bind(j, fma);
  37     bar bnf = bind(n, fma);
  38
  39     print("fmap Just false: ", jf.fst, jf.snd, "\n");
  40     print("fmap Nothing: ", nf.fst, "\n");
  41
  42     print("bind Just False: ", bjf.fst, bjf.snd, "\n");
  43     print("bind Nothing: ", bnf.fst, "\n");
  44 }