-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA512 On 20/10/15 13:23, Joachim Breitner wrote:
This is expected. If there is any ? in the type, then GHC wants all variables to be properly bound. This is not related to whether a is used or not: I see. I still think the error message could be more specific though...
Furthermore the first code snippet I gave also strikes me as nonsensical; if someone has code like that, it is probably the result of a typo. Should we really permit it? Is there a motivating reason for permitting it? If no, I suggest it be an error. (My understanding of the 2010 report was that it *isn't* permitted, but perhaps I read it wrong.) You mean type constraints involving an otherwise unused type variable? This seems to be unrelated to the question of foralls. It is. It just hit me while reading through my email before sending it.
But indeed the 2010 report writes “The context cx must only contain type variables referenced in t.” https://www.haskell.org/report/node/2010/haskellch4.html#x10-660004.1.3
and even with -XHaskell2010, this is accepted. Then I understood it correctly. I think we should fix this.
Might be worth a ticket, although I would not be surprised if some of the type class hackers around here finds a use case for this :-) If there is a usecase for this, it could be a pragma. GHC should not accept it by default though.
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