expr/exist/exist.icl

   1 implementation module exist
   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 class eval m  :: m -> ([(String, Int)] -> Either String Int)
  13 class print m :: m -> String
  14
  15 instance eval Expr
  16 where
  17         eval (Lit i) = const $ pure i
  18         eval (Var s) = maybe (Left $ "Undefined variable " +++ s) Right o lookup s
  19         eval (a +. b) = \s->liftM2 (+) (eval a s) (eval b s)
  20         eval (Ext e) = eval e
  21
  22 instance print Expr
  23 where
  24         print (Lit i) = toString i
  25         print (Var s) = s
  26         print (a +. b) = print a +++ " + " +++ print b
  27         print (Ext e) = print e
  28
  29 Start = eval (Lit 5 +. Lit 37) []