#4516: Over eager non-exhaustive pattern match warnings in lambda expressions.
---------------------------------+------------------------------------------
Reporter: benl | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.0.1 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: Incorrect warning at
compile-time
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When I write:
{{{
let thing = (\(Just x) -> x) whatever
in rest
}}}
GHC shouldn't complain about non-exhaustive pattern matches for this code,
because the form of the expression makes it obvious that some patterns are
not being matched. The above fragment should behave like:
{{{
let thing = let Just x = whatever in x
in rest
}}}
This pattern is also non-exhaustive, but that's obvious from the form of
the expression. I use both of these forms all the time because the
alternative of using something like {{{fromJust}}} gives bad error
messages when it fails.
Here's a more concrete example from DDC:
{{{
let vks' = map (\(T.TVar k (T.UVar v')) -> (v', k))
$ map toCoreT cts
}}}
To update my code to work with 7.0.1 I have to change all of these to:
{{{
let vks' = map (\tt -> let T.TVar k (T.UVar v') = tt in (v', k))
$ map toCoreT cts
}}}
This is longer, uglier, and requires the introduction of the dummy
variable {{{tt}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4516>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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