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TL;DR: Rewrite rules should be able to match instance methods.

I know that the interaction of rewrite rules and classes/instances/methods is unsatisfying, and probably will be for the forseeable future given the current design. But we might still improve little bits.

Consider this code:

{-# RULES
    "[Integer] Eq Refl" forall (xs :: [Integer]). xs == xs = True
#-}

This is the best I can to express the intention of “dear compile, this is a rule for the equality on lists if the elements are integers”. But what I get is

"[Integer] Eq Refl" [ALWAYS]
    forall ($dEq_a1Jv :: Eq [Integer]) (xs_a1Hd :: [Integer]).
      == @ [Integer] $dEq_a1Jv xs_a1Hd xs_a1Hd
      = GHC.Types.True

which is a rule about the method == applied to any evidence of equality about lists. This works in the most obvious cases, such as

alwaysTrue :: [Integer]-> Bool
alwaysTrue xs = xs == xs

but it does not fire with

delayedId :: a -> a
delayedId x = x
{-# INLINE [0] delayedId #-}

alwaysTrue :: [Integer]-> Bool
alwaysTrue xs = xs == delayedId xs
{-# NOINLINE alwaysTrue #-}

The reason is that directly after the simplifier, in the former case, the Core looks like this

$dEq_a1HH :: Eq [Integer]
[LclId, Str=DmdType]
$dEq_a1HH = GHC.Classes.$fEq[] @ Integer integer-gmp-1.0.0.1:GHC.Integer.Type.$fEqInteger

alwaysTrue [InlPrag=NOINLINE] :: [Integer] -> Bool
[LclIdX, Str=DmdType]
alwaysTrue = \ (xs_aGT :: [Integer]) -> == @ [Integer] $dEq_a1HH xs_aGT xs_aGT

which matches the rule, but in the latter case, by the time the delayedId is out of the way, we have

alwaysTrue [InlPrag=NOINLINE] :: [Integer] -> Bool
[LclIdX, Arity=1, Str=DmdType <S,U>]
alwaysTrue =
  \ (xs_aGT :: [Integer]) ->
    GHC.Classes.$fEq[]_$c==
      @ Integer
      integer-gmp-1.0.0.1:GHC.Integer.Type.$fEqInteger
      xs_aGT
      xs_aGT

One possible way forward would be to simplify the wanted constraints on the LHS of the rule using the instances in scope:

"[Integer] Eq Refl" [ALWAYS]
    forall (xs_a1Hd :: [Integer]).
    GHC.Classes.$fEq[]_$c==
      @ Integer
      integer-gmp-1.0.0.1:GHC.Integer.Type.$fEqInteger
      xs_a1Hd
      xs_a1Hd
    = True

This might be tricky, of course, as this requires not only help from the type checker, but also some careful tuned simplification of the LHS afterwards (e.g. unfold dictionary accessors)).

Trac metadata

Trac field	Value
Version	8.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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