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

23 Mar 2015

      #10183: Warning for redundant constraints: interaction with pattern matching
-------------------------------------+-------------------------------------
        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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