old/expr/deep.icl

   1 module deep
   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 :: Expr
  13         = Lit Int
  14         | Var String
  15         | (+.) infixl 6 Expr Expr
  16
  17 eval :: Expr -> ([(String, Int)] -> Either String Int)
  18 eval (Lit i) = const $ pure i
  19 eval (Var s) = maybe (Left $ "Undefined variable " +++ s) Right o lookup s
  20 eval (a +. b) = \s->liftM2 (+) (eval a s) (eval b s)
  21
  22 print :: Expr -> String
  23 print (Lit i) = toString i
  24 print (Var s) = s
  25 print (a +. b) = print a +++ " + " +++ print b
  26
  27 Start = eval (Lit 5 +. Lit 37) []