#2006: unreachable GADT pattern clauses show up as warnings with -Wall
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Reporter: ryani | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.8.1
Severity: normal | Keywords:
Difficulty: Unknown | Testcase:
Architecture: Unknown | Os: Unknown
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In the following code, there is a line commented out. With it commented
out, I get the following warning:
{{{
patternbug.hs:10:0:
Warning: Pattern match(es) are non-exhaustive
In the definition of `interpret': Patterns not matched: EVar
}}}
With it uncommented, I get the following compiler error:
{{{
patternbug.hs:11:10:
Inaccessible case alternative: Can't match types `()' and `(a, vs)'
In the pattern: EVar
In the definition of `interpret':
interpret EVar = error "unreachable"
}}}
The exhaustive pattern-matching algorithm should ignore unreachable cases
in its analysis.
{{{
{-# OPTIONS_GHC -Wall #-}
{-# LANGUAGE GADTs #-}
module PatternBug where
data Expr a vs where
EPrim :: String -> a -> Expr a vs
EVar :: Expr a (a,vs)
interpret :: Expr a () -> a
interpret (EPrim _ a) = a
-- interpret EVar = error "unreachable"
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2006>
GHC <http://www.haskell.org/ghc/>
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