Confusing error message when universally quantifying unused type variables

[Alexander Berntsen]

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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.
- -- 
Alexander
alexander at plaimi.net
https://secure.plaimi.net/~alexander
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