I expect this is a result of https://ghc.haskell.org/trac/ghc/ticket/8883, but even so it's not clear to me whether the error is correct. It would be nice if GHC printed the type it inferred for succ' in this kind of situation.

Regards,
Reid Barton

On Tue, Jun 10, 2014 at 7:55 AM, Gabor Greif <ggreif@gmail.com> wrote:

Devs,

as of recently GHC HEAD stopped building the diagrams library. Several
prerequisite libs also fail to compile. A specific error message
appears in an attoparsec issue
<https://github.com/bos/attoparsec/issues/67>, for which I have
submitted a workaround. But as Herbert cautiously points out, this
could be a recent GHC bug surfacing.

The symptoms of the bug seem to be that GHC complains about the usage
of (~) type equality operation, but there is no reference to that type
operator in the source code. To work around the problem it suffices to
add -XGADTs or -XTypeFamilies.

Does this ring any bells? Can automatized tests catch such things in the future?

Cheers,

Gabor
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