arising from the dependency ... in the instance declaration

Iavor Diatchki iavor.diatchki at
Tue Dec 3 22:12:36 UTC 2013


oh, I didn't realize that you meant using HEAD.

I'd guess that the new behavior is a result of the changes Simon made to
the constraint solver a little while ago (maybe about a month ago?).
I haven't had a chance to look at the code yet, but I believe he eliminated
some constraint interactions, which did not contribute towards
type-checking more programs, but lead to more confusing error messages.

By the way, I am not sure that this is a regression---I kind of like new
the new behavior.  For my second example we get:

    No instance for (C Bool Float) arising from a use of ‛op’
    In the expression: op True :: Float
    In an equation for ‛test’: test = op True :: Float

This tells us exactly what the problem is, namely the missing instance,
without going into stuff about functional dependencies.  I guess we _could_
also say that such an instance can't exist because of the other instance
and the functional dependency, but I am not sure how relevant that is...


On Tue, Dec 3, 2013 at 1:40 PM, Joachim Breitner
<mail at>wrote:

> Hi,
> Am Dienstag, den 03.12.2013, 11:08 -0800 schrieb Iavor Diatchki:
> > Here is a piece of code that produces the desired error:
> >
> > [..]
> >
> > The `DataKinds` and `KindSigantures` are only used to match your
> > example closely.
> > Here is a simpler version that causes essentially the same error:
> >
> > [..]
> >
> thanks for these examples. Both, though, give different error messages
> in GHC HEAD. I’m more and more confident that this is an inadvertent
> error message, see
> Once that is sorted out, I’ll make sure these examples (or at least a
> representative one) makes it into the testsuite.
> Greetings,
> Joachim
> --
> Joachim “nomeata” Breitner
>   mail at joachim-breitner.de
>   Jabber: nomeata at  • GPG-Key: 0x4743206C
>   Debian Developer: nomeata at
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