appendix/lst/expr_gadt.hs

   1 module ExprGADT where
   2 import Prelude hiding (print)
   3
   4 data Expr a where
   5     Lit :: Show a => a -> Expr a
   6     Add :: Num a  => Expr a -> Expr a -> Expr a
   7     Eq  :: Eq e   => Expr e -> Expr e -> Expr Bool
   8
   9 eval :: Expr a -> a
  10 eval (Lit e)   = e
  11 eval (Add l r) = eval l + eval r
  12 eval (Eq  l r) = eval l == eval r
  13
  14 print :: Expr a -> String
  15 print (Lit e)   = show e
  16 print (Add l r) = print l ++ "+"  ++ print r
  17 print (Eq  l r) = print l ++ "==" ++ print r