[Haskell-cafe] known, I know: class contexts and mutual recursion
ctm at cs.nott.ac.uk
Mon Dec 4 13:26:31 EST 2006
Iavor Diatchki wrote:
>> I just tripped over the "Contexts differ in length" error message. I
>> know it's not a new problem, but I thought I'd enquire as to its status.
> I have run into that a number of times. There aren't any technical
> issues with solving it, in fact, depending on how one implements the
> type checker, it is more work to implement this extra restriction
Yes, that's what I would expect. It's an inevitable but not threatening
consequence of polymorphic recursion. It rather surprised me that just
telling the machine what my plan was (something I always do anyway)
produced such an objection.
> It is proposed for fixing in Haskell' but I was surprised that it did
> not seem to appear on the "definitely in" list (as far as I can
> recall---I might be wrong).
From what I can tell, it's "probably in". Strikes me as fairly
essential. I don't expect artificial restrictions to be placed on the
types of my programs when I am willing to make those types explicit.
> Hopefully the report will be fixed
> though. Otherwise, as Ian mentioned, it works in GHC 6.6 with
> -fglasgow-exts. Also, there should be no problems with higher-ranked
> types etc.
Thanks for the tip, Ian. I guess I shouldn't put off installing 6.6 any
All the best
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