Custom Query

Reported by SamAnklesaria, 14 months ago.

The following rewrite rule fires for a normal function, but fails to fire as the method of a typeclass.

{-# OPTIONS_GHC -O -ddump-rule-firings #-}
module RewriteProblems where

{-# RULES
"rewrite/class"    forall a. id1 (id1 a) = RDUnit
"rewrite/static"   forall a. id1' (id1' a) = RDUnit
   #-}

class Ider a where
    id1 :: a -> a

data RewriteD = RDUnit

instance Ider RewriteD where
    {-# INLINE[1] id1 #-}
    id1 a = RDUnit

classTest :: RewriteD
classTest = id1 (id1 RDUnit)

staticTest :: RewriteD
staticTest = id1' (id1' RDUnit)

{-# INLINE[1] id1' #-}
id1' :: RewriteD -> RewriteD
id1' a = RDUnit

Reported by SamAnklesaria, 12 months ago.

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?.

Reported by WouterSwierstra, 6 years ago.

I'd propose to add a Data.Stream library to base. Data.Stream should be fairly uncontroversial. The module consists largely of a reimplimentation of several functions from Data.List to create, modify, and manipulate infinite lists. There is already a package on Hackage:

 http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Stream-0.1

I've tried to include Haddock-comments, very similar to those for Data.List. As working with infinite lists can be tricky, I've included warnings clearly stating which functions can diverge.

I am aware that the name may cause confusion with Data.List.Stream. I'd welcome suggestions to avoid causing confusion.