[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
              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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 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.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10183>
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