#7156: "Pattern match on GADT" error for non-GADT
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Reporter: ryani | Owner:
Type: bug | Status:
new
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version:
7.0.4
Keywords: GADTs, TypeFamilies, ExistentialQuantification | Os:
Unknown/Multiple
Architecture: Unknown/Multiple | Failure:
None/Unknown
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Comment(by ryani):
I expect that if you can define a data type with particular extensions,
you should be able to pattern match on that data type successfully. Or
else, what's the point of defining it? This is a failure in a single
module, not two modules defined with different extensions, which surprised
me!
There extensions being used here, EqualityConstraints (via TypeFamilies)
and ExistentialQuantification, are well known to be equivalent in power to
GADTs in defining types. But, I guess there is extra type system
machinery to deal with the {{{ T a -> a }}} vs {{{ T a -> () }}} problem
that is currently only enabled with GADTs?
It seems like equality constraints require that same machinery; at least
if they are able to be brought into scope via pattern matching.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7156#comment:3>
GHC <http://www.haskell.org/ghc/>
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