#7541: Unavoidable duplicate constraint warning
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Reporter: blamario | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.6.1 | Keywords:
Os: Unknown/Multiple | Architecture:
Unknown/Multiple
Failure: Incorrect warning at compile-time | Blockedby:
Blocking: | Related:
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The "duplicate constraint" warning can be triggered in a context where
it's impossible to remove it, because the constraints come with a type
synonym.
The following standalone little module reproduces the problem:
{{{
{-# LANGUAGE FlexibleContexts, Rank2Types #-}
module Test where
type Constrained x y r = (Eq x, Eq y) => x -> y -> r
f :: Constrained String String ()
f = undefined
}}}
The warning is
{{{
Test.hs:7:6: Warning:
Duplicate constraint(s): Eq String
In the type signature for `f': f :: Constrained String String ()
}}}
The warning is present in 7.4.1 and 7.6.1, and presumably in the versions
between as well.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7541>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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