Jules Bean wrote:
Peter Padawitz wrote:
So the fundep would solve the problem.
But, actually, it doesn't :-(
But actually, it does!
Indeed... Sorry, I think I left intE out of the cycle. This might be the reason why it did not work before.
Ben Franksen's answer from yesterday compiles fine for me if I add the missing fundep, block -> command.
Your original code compiles without error, given the fundep. Exact code I compiled attached at the bottom of this document. You may have to repair long lines!
Incidentally, I question why the "compFoo" are methods. Why not just make them polymorphic functions? They don't look like you expect instances to change them. The code continues to compile if I make them functions and amend their signatures as required.
I put compFoo into the class for the same reason why /= is part of the class Eq: both functions are unique as soon as the others have been instantiated.
{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-}
type Block = [Command] data Command = Skip | Assign String IntE | Cond BoolE Block Block | Loop BoolE Block data IntE = IntE Int | Var String | Sub IntE IntE | Sum [IntE] | Prod [IntE] data BoolE = BoolE Bool | Greater IntE IntE | Not BoolE
class Java block command intE boolE | block -> command, command -> intE, intE -> boolE, boolE -> block where block_ :: [command] -> block skip :: command assign :: String -> intE -> command cond :: boolE -> block -> block -> command loop :: boolE -> block -> command intE_ :: Int -> intE var :: String -> intE sub :: intE -> intE -> intE sum_ :: [intE] -> intE prod :: [intE] -> intE boolE_ :: Bool -> boolE greater :: intE -> intE -> boolE not_ :: boolE -> boolE compBlock :: Block -> block compBlock = block_ . map compCommand compCommand :: Command -> command compCommand Skip = skip compCommand (Assign x e) = assign x (compIntE e) compCommand (Cond be cs cs') = cond (compBoolE be) (compBlock cs) (compBlock cs') compCommand (Loop be cs) = loop (compBoolE be) (compBlock cs) compIntE :: IntE -> intE compIntE (IntE i) = intE_ i compIntE (Var x) = var x compIntE (Sub e e') = sub (compIntE e) (compIntE e') compIntE (Sum es) = sum_ (map compIntE es) compIntE (Prod es) = prod (map compIntE es) compBoolE :: BoolE -> boolE compBoolE (BoolE b) = boolE_ b compBoolE (Greater e e') = greater (compIntE e) (compIntE e') compBoolE (Not be) = not_ (compBoolE be)