#2079: GHC 6.8.* doesn't recognize type variables
-------------------------------+--------------------------------------------
Reporter: daniel.is.fischer | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.8.2
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Testcase: | Architecture: x86
Os: Linux |
-------------------------------+--------------------------------------------
Changes (by simonpj):
* difficulty: => Unknown
Comment:
This is really an error message bug. Without `-XFlexibleInstances`, the
restriction is that there must be no repeated type variables in the
instance head. Whether this is a useful restriction to separate from
multi-parameter type classes is moot. As it stands I agree it's
confusing. I can think of three possibilities:
* We could make `-XMultiParamTypeClasses` imply `-XFlexibleInstances`.
* We could collapse the two options into one option.
* We could leave the language pragmas unchanged, but change the error
message to say "...where the a1..an are type *variables*, and each type
variable appears at most once in the instance head". Would that have
helped?
Of these, the third is least likely to cause trouble others.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2079#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
_______________________________________________
Glasgow-haskell-bugs mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
