On Dec 7, 2007 5:57 PM, Peter Padawitz
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 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 kcompIntE es)
compBoolE :: BoolE -> boolE compBoolE (BoolE b) = boolE_ b compBoolE (Greater e e') = greater (compIntE e) (compIntE e') compBoolE (Not be) = not_ (compBoolE be)
I'm not sure what this is worth, since you seem to have your mind set on using this strange four-parameter type class. You can keep most of the design advantages of using this type at the "cost" of being more explicit if you factor it into a data type yourself:
data Java block command intE boolE = Java { block_ :: [command] -> block , skip :: command , assign :: String -> intE -> command , ... , compBlock :: Block -> block , ... }
For your default implementations:
defCompBlock :: Java block command intE boolE -> Block -> block defCompBlock self = block_ self . map (compCommand self)
.. etc
Then to define an example instance:
javaAST :: Java Block Command IntE BoolE javaAST = Java { block_ = Block , ... , compBlock = defCompBlock javaAST , ... }
Your type errors will be resolved because you are saying explicitly which instance to use by passing the instance data structure you want explicitly. Luke