rank2/test.icl

   1 module test
   2
   3 import StdEnv
   4 import Data.Maybe
   5 import Data.Functor
   6 import Control.Applicative
   7
   8 class expr v where
   9         lit :: i -> v i | toString i
  10         (+.) infixl 6 :: (v i) (v i) -> v i | + i
  11
  12 instance + (v a) | expr v & + a where
  13         + l r = l +. r
  14
  15 eval :: (Maybe a) -> Maybe a
  16 eval x = x
  17 instance expr Maybe where
  18         lit i = Just i
  19         +. x y = (+) <$> x <*> y
  20
  21 :: Print a =: Print String
  22 print :: (Print a) -> String
  23 print (Print a) = a
  24 instance expr Print where
  25         lit i = Print (toString i)
  26         +. (Print l) (Print r) = Print (l +++ "+" +++ r)
  27
  28 printEval :: (A.v: v a | expr v) -> (Maybe a, String)
  29 printEval f = (eval f, let (Print p) = f in p)
  30
  31 //Mag niet
  32 Start :: (Maybe Int, String)
  33 Start = printEval (lit 4 +. lit 38)
  34
  35 //Mag wel
  36 //Start = let (Print f) = lit 4 + lit 38 in f