On Fri, Jul 22, 2011 at 11:12 AM, Serguey Zefirov
--------------------------------------------------------------------------------------------------------------------- {-# LANGUAGE GADTs, TypeFamilies #-}
class CPU cpu where type CPUFunc cpu
data Expr cpu where EVar :: String -> Expr cpu EFunc :: CPU cpu => CPUFunc cpu -> Expr cpu
class CPU cpu => FuncVars cpu where funcVars :: CPUFunc cpu -> [String]
exprVars :: FuncVars cpu => Expr cpu -> [String] exprVars (EVar v) = [v] -- an offending line: exprVars (EFunc f) = funcVars f ---------------------------------------------------------------------------------------------------------------------
I tried to split creation and analysis constraints. CPU required for creation of expressions, FuncVars required for analysis. It all looks nice but didn't work.
(In our real code EVar is slightly more complicated, featuring "Var cpu" argument)
It looks like GHC cannot relate parameters "inside" and "outside" of GADT constructor.
Not that I hesitate to add a method to a CPU class, but I think it is not the right thing to do. So if I can split my task into two classes, I will feel better.
GHC cannot decide what instance of FuncVars to use. The signature of funcVars is: funcVars :: FuncVars cpu => CPUFunc cpu -> [String] This does not take any arguments that allow cpu to be determined. For instance, if there were instances (rolling them into one declaration for simplicity): instance FuncVars Int where type CPUFunc Int = Int ... instance FuncVars Char where type CPUFunc Char = Int Then GHC would see that CPUFunc cpu = Int, but from this, it cannot determine whether cpu = Int or cpu = Char. CPUFunc is not (necessarily) injective. Making CPUFunc a data family as Felipe suggested fixes this by CPUFunc essentially being a constructor of types, not a function that computes. So it would be impossible for CPUFunc a = CPUFunc b unless a = b. Also, if you have a class whose only content is an associated type, there's really no need for the class at all. It desugars into: type family CPUFunc a :: * class CPU a So it's just a type family and an empty class, which will all have exactly the same cases defined. You could instead use just the family. -- Dan