[GHC] #10183: Warning for redundant constraints: interaction with pattern matching

[GHC]

#10183: Warning for redundant constraints: interaction with pattern matching
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        Reporter:  simonpj           |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.8.4
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Description changed by simonpj:

Old description:

> Herbert gives this example:
> {{{
> {-# LANGUAGE GADTs, DataKinds, TypeOperators, UnicodeSyntax #-}
>
> import GHC.TypeLits
>
> data List l t where
>      Nil  ∷ List 0 t
>      (:-) ∷ t → List l t → List (l+1) t
>
> head' ∷ (1<=l) ⇒ List l t → t
> head' (x :- _) = x
> }}}
> We get the warning
> {{{
>   typelits1.hs:9:9: Warning:
>       Redundant constraint: 1 <= l
>       In the type signature for: head' ∷ (1 <= l) ⇒ List l t → t
> }}}
> But of course the warning is not redundant if we want to exclude the
> `(head' Nil)` case.
>
> Here's an example that doesn't depend on `TypeLits`:
> {{{
> data T a where
>   TT :: T True
>   TF :: T False
>
> f :: T True -> Bool
> f TT = True
>
> g :: (a ~ True) => T a -> Bool
> g TT = True
> }}}
> Here `f` compiles fine, but `g` warns about a redundant constraint!  And
> indeed the evidence for `(a~True)` is not used in the code for `g`.  But
> that evidence is used to prove that the un-matched pattern `(g TF)` is
> unreachable.
>
> I'm not sure how to fix this.  Bother.

New description:

 Herbert gives this example:
 {{{
 {-# LANGUAGE GADTs, DataKinds, TypeOperators, UnicodeSyntax #-}

 import GHC.TypeLits

 data List l t where
      Nil  ∷ List 0 t
      (:-) ∷ t → List l t → List (l+1) t

 head' ∷ (1<=l) ⇒ List l t → t
 head' (x :- _) = x
 }}}
 We get the warning
 {{{
   typelits1.hs:9:9: Warning:
       Redundant constraint: 1 <= l
       In the type signature for: head' ∷ (1 <= l) ⇒ List l t → t
 }}}
 But of course the warning is not redundant if we want to exclude the
 `(head' Nil)` case.

 Here's an example that doesn't depend on `TypeLits`:
 {{{
 data T a where
   TT :: T True
   TF :: T False

 f :: T True -> Bool
 f TT = True

 g :: (a ~ True) => T a -> Bool
 g TT = True
 }}}
 Here `f` compiles fine, but `g` warns about a redundant constraint, even
 though the two types are isomorphic!  And indeed the evidence for
 `(a~True)` is not used in the code for `g`.  But that evidence is used to
 prove that the un-matched pattern `(g TF)` is unreachable.

 I'm not sure how to fix this.  Bother.

--

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10183#comment:1>
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