Subclass Specialization in Rewrite Rules

Rewrite rules can define a specialized version of a method for some specific datatype, but they cannot currently define a specialized version of a method for some specific superclass.

class ClassOne a where
   classOneOp :: a -> a 

class ClassOne a => ClassTwo a where
   classTwoOp :: a -> a

data ClassInstance = ...

instance ClassOne ClassInstance where
   classOneOp = ...

specialFunc :: ClassInstance -> ClassInstance
specialFunc = ...

{-# RULES
"willcompile"        forall i. classOneOp i = specialFunc
"wontcompile"      forall i. classOneOp i = classTwoOp i
   #-}

Although we can specialize classOneOp for ClassInstance, we can't do so for those instances of ClassOne that are also instances of ClassTwo.

Trac metadata

Trac field	Value
Version	7.4.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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