old/expr/class.icl

   1 implementation module class
   2
   3 import StdEnv
   4 import Data.Func
   5 import Data.Functor
   6 import Data.Either
   7 import Data.List
   8 import Data.Maybe
   9 import Control.Applicative
  10 import Control.Monad
  11
  12 //Easy to add functionality
  13 //Easy to add backends
  14 //Type safety as in GADTs
  15
  16 class lit m :: Int -> m
  17 class var m :: String -> m
  18 class (+.) infixl 6 m :: m m -> m
  19
  20 :: Eval :== ([(String, Int)] -> Either String Int)
  21 instance lit Eval where lit i = pure (pure i)
  22 instance var Eval where var s = maybe (Left $ "Undefined variable " +++ s) Right o lookup s
  23 instance +. Eval where (+.) a b = \s->liftM2 (+) (a s) (b s)
  24
  25 :: Print :== String
  26 instance lit Print where lit i = toString i
  27 instance var Print where var s = s
  28 instance +. Print where (+.) a b = a +++ " + " +++ b
  29
  30 Start :: Eval
  31 Start = (lit 5 +. lit 37)