HEADS-UP: diagrams does not compile with HEAD (regressions)

Simon Peyton Jones simonpj at microsoft.com
Tue Jun 10 15:02:59 UTC 2014

I suspect that the issue is the following: GHC is inferring a type for a function that could not be written by the user, at least not without -XTypeFamilies or -XGADTs, something like
	f :: (a ~ F b) => blah blah

It's a good principle that if GHC infers a type, then it should be ok to put that type in manually.  But that means that you need -XTypeFamilies even for the inferred type.

So unless I'm misunderstanding the issue, it's not a bug but a feature.  It is a change, certainly, and one that makes GHC a bit more picky.

Can you update diagrams to include type signatures or -XGADTs/TypeFamilies?



| -----Original Message-----
| From: ghc-devs [mailto:ghc-devs-bounces at haskell.org] On Behalf Of Gabor
| Greif
| Sent: 10 June 2014 12:55
| To: ghc-devs
| Subject: HEADS-UP: diagrams does not compile with HEAD (regressions)
| 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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| ghc-devs mailing list
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