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

[GHC]

#10183: Warning for redundant constraints: interaction with pattern matching
-------------------------------------+-------------------------------------
        Reporter:  simonpj           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.11
      Resolution:                    |             Keywords:  redundant-
                                     |  constraints, GADTs
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by thomie):

 * keywords:   => redundant-constraints, GADTs
 * failure:  None/Unknown => Incorrect warning at compile-time


@@ -23,1 +23,1 @@
- Here's an example that doesn't depend on `TypeLits`:
+ Here's an example that only depends on `GADTs`:
@@ -25,0 +25,1 @@
+ {-# LANGUAGE GADTs #-}
@@ -26,2 +27,2 @@
-   TT :: T True
-   TF :: T False
+   TT :: T Bool
+   TF :: T Int
@@ -29,1 +30,1 @@
- f :: T True -> Bool
+ f :: T Bool -> Bool
@@ -32,1 +33,1 @@
- g :: (a ~ True) => T a -> Bool
+ g :: (a ~ Bool) => T a -> Bool
@@ -37,1 +38,1 @@
- `(a~True)` is not used in the code for `g`.  But that evidence is used to
+ `(a~Bool)` is not used in the code for `g`.  But that evidence is used to

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 only depends on `GADTs`:
 {{{
 {-# LANGUAGE GADTs #-}
 data T a where
   TT :: T Bool
   TF :: T Int

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

 g :: (a ~ Bool) => 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~Bool)` 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:4>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler