[Haskell-cafe] type class question

[Brent Yorgey]

On Dec 3, 2007 7:43 AM, Peter Padawitz <peter.padawitz at 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
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