[Haskell-cafe] type class question

[Peter Padawitz]

Yes, the recursive calls of compCommand are supposed to be calls of 
compBlock.

The intention of the program is a generic evaluator comp... of 
Sigma-terms in arbitrary Sigma-algebras. The signature Sigma is given by 
the first 4 types (and the corresponding functions in the class 
declaration), the terms are the objects of the types, and the algebras 
are the class instances.

The problem with my implementation in terms of multiple-parameter 
classes seems to be - I conclude this from Ryan's comment - that  the 
intended dependency among the parameters is not reflected here. But what 
are the alternatives? Roughly said, I need a construct that allows me 
gather several type variables such that an instance is always an 
instance of all of them.

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