#2905: require -XGADTs in order to pattern-match GADTs
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Reporter: guest | Owner:
Type: proposal | Status: new
Priority: normal | Component: Compiler (Type checker)
Version: 6.10.1 | Severity: normal
Keywords: | Testcase:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
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Without -XGADTs, pattern-matching GADTs causes weird typechecking problems
(due to the lack of implied -XRelaxedPolyRec), see this thread
http://thread.gmane.org/gmane.comp.lang.haskell.glasgow.user/16064 , and
some history in bug #2004
I propose that pattern-matching against any constructor that uses
sufficient GADT features that it's particularly confused without
-XRelaxedPolyRec, should require -XGADTs
(I don't understand the issue well enough to know if what I'm saying makes
sense. I guess ordinary data and mere existentials don't need it, even
when defined with GADT syntax...?)
Alternately we could just require -XRelaxedPolyRec, but that seems even
more confusing (and it wouldn't help make code portable to other
compilers, as GADTs are definitely being used). Or if Haskell-prime ever
comes around and makes -XRelaxedPolyRec default (is it planning to?), then
the issue might be moot. I don't think the argument (against requiring
-XGADTs) by parallel to -XOverlappingInstances is particularly strong...
-XOverlappingInstances has an implicit effect on a module's class
definitions... also I don't necessarily agree with that we made that
decision for existing flags either :-)
--Isaac Dupree
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2905>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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