Unnecessary equality superclass
Consider
{-# LANGUAGE MultiParamTypeClasses, GADTs, TypeOperators #-}
module Foo where
class (a ~ b) => C a b where
op :: a -> a -> b
f :: C a b => a -> b
f x = op x x
If you compile this you'll end up with
f = \ (@ a_a1dk)
(@ b_a1dl)
($dC_a1dn :: C a_a1dk b_a1dl)
(eta_B1 :: a_a1dk) ->
case GHC.Types.heq_sel
@ *
@ *
@ a_a1dk
@ b_a1dl
((Foo.$p1C @ a_a1dk @ b_a1dl $dC_a1dn)
`cast` (Data.Type.Equality.N:~[0] <*>_N <a_a1dk>_N <b_a1dl>_N
:: (a_a1dk ~ b_a1dl) ~R# (a_a1dk ~~ b_a1dl)))
of co_a1dw
{ __DEFAULT ->
op @ a_a1dk @ b_a1dl $dC_a1dn eta_B1 eta_B1
}
What is that unused heq_sel
doing?
It happens during solving
- We have
[G] a ~ b
- And, by superclasses we have
[G] a ~# b
- We use this to rewrite
a
tob
in both givens and wanteds
which is all fine, but we end up with
f = \ (@ a_a1dk) (@ b_a1dl) ($dC_a1dn :: C a_a1dk b_a1dl) ->
case GHC.Types.heq_sel
@ *
@ *
@ a_a1dk
@ b_a1dl
(Data.Type.Equality.$p1~
@ * @ a_a1dk @ b_a1dl (Foo.$p1C @ a_a1dk @ b_a1dl $dC_a1dn))
of co_a1dw
{ __DEFAULT ->
\ (x_axX :: a_a1dk) ->
op
@ a_a1dk
@ b_a1dl
(($dC_a1dn
`cast` ((C co_a1dw <b_a1dl>_N)_R
:: C a_a1dk b_a1dl ~R# C b_a1dl b_a1dl))
`cast` ((C (Sym co_a1dw) <b_a1dl>_N)_R
:: C b_a1dl b_a1dl ~R# C a_a1dk b_a1dl))
x_axX
x_axX
}
Notice that co_a1dw
is used. But we are just casting and casting back, so it ends up as Refl and co_a1dw
is unused.
Nothing is wrong here, but it seems inelegant
- Can we discard that
heq_sel
? Perhaps we can declare that dictionary-valued terms are always treated strictly (#2439), so that(heq_sel ...)
is guaranteed non-bottom, and we can discard the case. - Or maybe we can expand the given superclasses less aggressively, so that the equality isn't exposed until necessary. But see
Note [Eagerly expand given superclasses]
inTcCanonical
.
Trac metadata
Trac field | Value |
---|---|
Version | 8.4.3 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |