#595: Overhaul GHC's overlapping/non-exhaustive pattern checking
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Reporter: simonmar | Owner:
Type: task | Status: new
Priority: normal | Milestone: _|_
Component: Compiler | Version: None
Resolution: None | Keywords: warnings
Os: Unknown/Multiple | Architecture:
Unknown/Multiple
Failure: Incorrect warning at compile-time | Difficulty: Difficult
(2-5 days)
Testcase: N/A | Blockedby:
Blocking: | Related:
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Comment(by simonpj):
No, no one is working on it, as far as I know, and I would love someone to
pay attention to it. There are a bunch of related tickets: at least #595,
#5728, #3927, #5724, #5762, #4139. It has the great merit of being a
nicely-separable task, one that doesn't interact with the rest of GHC.
There is a good literature to guide you:
* Several papers by Luc Maranget, including a JFP paper
[http://moscova.inria.fr/~maranget/papers/warn/index.html Warnings for
patttern matching]
* Neel Krishnaswami [http://www.cs.cmu.edu/~neelk/pattern-popl09.pdf
Focusing on pattern matching]
However these papers focus entirely on a "block" of pattern matching, for
example, a function defined by pattern matching. Many of the tickets go
further:
* Consider
{{{
f xs@(y:ys) = .....case xs of { (y:ys) -> ... }
}}}
The inner pattern match should not complain about a missing nil case.
You may think this is a silly program, but it's less silly when record
updates are concerned.
* Matches that involve GADTs. Here things are a bit more complicated,
and involve figuring out what branches are inaccessible because of type
constraints. It's not that hard, but generating good warnings goes beyond
what the literature covers.
I don't want to supervise this, but would would be happy to offer advice
and guidance about GHC aspects, if someone else would lead.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/595#comment:20>
GHC <http://www.haskell.org/ghc/>
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