#4439: GHC doesn't detect trivial complete pattern with the combination of
ExistentialQuantification and ViewPatterns
---------------------------------+------------------------------------------
Reporter: batterseapower | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 6.12.3 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: Incorrect warning at
compile-time
---------------------------------+------------------------------------------
This program exhibits the behaviour:
{{{
{-# LANGUAGE ViewPatterns, ExistentialQuantification #-}
{-# OPTIONS_GHC -fwarn-incomplete-patterns #-}
data Moo = Moo (Char -> Int)
spqr (Moo _) = undefined -- Exhaustive pattern
foo (id -> Moo _) = undefined -- Exhaustive pattern
data Exists = forall a. Exists (a -> Int)
bar (Exists _) = undefined -- Exhaustive pattern
baz (id -> Exists _) = undefined -- "Warning: Pattern match(es) are non-
exhaustive"
}}}
GHCs output is puzzling:
{{{
$ ghc -c Exhaustive.hs
Exhaustive.hs:11:0:
Warning: Pattern match(es) are non-exhaustive
In the definition of `baz': Patterns not matched: _
}}}
It can detect exhaustivity if you use either existentials OR view patterns
by themselves, but not with the combination of the two together. Weird!
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4439>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
_______________________________________________
Glasgow-haskell-bugs mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
