expr/gadt.icl

   1 module gadt
   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 :: BM a b = {to :: a->b, fro :: b->a}
  13 bm = {to=id,fro=id}
  14 :: Expr a
  15         = Lit (BM a Int) Int
  16         | Var (BM a Int) String
  17         | E.e: Plus (BM a e) (Expr a) (Expr a) & + e
  18
  19 eval :: (Expr Int) -> ([(String, Int)] -> Either String Int)
  20 eval (Lit _ i) = const $ pure i
  21 eval (Var _ s) = maybe (Left $ "Undefined variable " +++ s) Right o lookup s
  22 eval (Plus _ a b) = \s->liftM2 (+) (eval a s) (eval b s)
  23
  24 print :: (Expr a) -> String
  25 print (Lit _ i) = toString i
  26 print (Var _ s) = s
  27 print (Plus _ a b) = print a +++ " + " +++ print b
  28
  29 Start = eval (Plus bm (Lit bm 5) (Lit bm 37)) []