On Dec 3, 2007 7:43 AM, Peter Padawitz <peter.padawitz@udo.edu> wrote:

What is wrong here? ghci tries (and fails) to deduce certain types for the comp functions that I did not expect.

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
   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 c c') = cond (compBoolE be) (compCommand c)
                                                          (compCommand c')
         compCommand (Loop be c)    = loop (compBoolE be) (compCommand c)-}

         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)


Well, first of all, the definition of compCommand should use calls to compBlock, not recursive calls to compCommand.  But that's not the main source of your problems.

What exactly are you trying to accomplish?  And why do you need a type class?

-Brent