expr/shallow.icl

   1 module shallow
   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         ([(String, Int)] -> Either String Int //Evaluator
  14         , String)                             //Printer
  15
  16 lit :: Int -> Expr
  17 lit i = (const $ pure i, toString i)
  18
  19 var :: String -> Expr
  20 var s =
  21         ( maybe (Left $ "Undefined variable " +++ s) Right o lookup s
  22         , s
  23         )
  24
  25 (+.) infixl 6 :: Expr Expr -> Expr
  26 (+.) (ea, pa) (eb, pb) =
  27         ( \s->liftM2 (+) (ea s) (eb s)
  28         , pa +++ " + " +++ pb
  29         )
  30
  31 print :: Expr -> String
  32 print (_, p) = p
  33
  34 eval :: Expr [(String, Int)]-> Either String Int
  35 eval (e, _) s = e s
  36
  37 Start = eval (lit 5 +. lit 37) []