#3691: Error message doesn't mention necessary extension in warning
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Reporter: arsenm | Owner:
Type: feature request | Status: reopened
Priority: normal | Milestone:
Component: Compiler | Version: 6.10.4
Resolution: | Keywords:
Difficulty: | Os:
Unknown/Multiple
Testcase: | Architecture:
Unknown/Multiple
Failure: Incorrect warning at compile-time |
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Comment (by isaacdupree):
Hmm, I think my reasoning is that we *should* discourage people from doing
the thing that (Simon's) proposed warning is about. Because it has the
potential to be confusing to people.
(In two circumstances.
"2" is: confusing to people who read the code, who know about
!ScopedTypeVariables (and may or may not understand the Haskell98
behavior!), and, being human, they don't remember to check the LANGUAGE
pragma/.cabal/makefile to discover correctly whether the extension is in
effect.
"1" is: confusing for people who copy/paste code or who turn on
!ScopedTypeVariables for some other reason, and then have to understand a
type error and fix it then by alpha-renaming. I think it would make more
sense to warn earlier and make the code get "fixed" while it's being
written. Also because, I think it's likely that an average/newish Haskell
programmer writing such code was trying to get scoped tyvar behavior and
didn't realize that they needed to turn on a flag.
)
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3691#comment:8>
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