Hello,
the following is not Haskell 98:
class C a
instance C (a,a)
Well, GHC allows this with the -fglasgow-exts flag. Surprisingly, I cannot
find a section in the GHC User's Guide which states that -fglasgow-exts can
be used to allow this kind of instance declarations. Well, in the section on
undecidable instances (7.4.4.3), one can read:
Note that instance heads may contain repeated type variables. For
example,
this is OK:
instance Stateful (ST s) (MutVar s) where ...
But one has to note that in this expample, the different occurences of s
aren't in the same type. Furthermore, multiple occurences of the same type
variable cannot result in undecidability (as far as I understand). The
above-mentioned statement talks about the things that are allowed *without*
-fallow-undecidable-instances, and premises that multiple occurences of a
type variable are okay.
So am I missing something?
Best wishes,
Wolfgang
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