Are there GHC extensions we'd like to incorporate wholesale?

[Richard Eisenberg]

Great questions. Here's my take:

For something to be incorporated into the standard, we'd need to be able to give a concrete, precise description of how the extension changes the set of correct Haskell programs. We also need to consider how the extension changes properties of the language, like predictability of performance or class coherence.

For the four you mention, here are my thoughts:

MultiParamTypeClasses: Can't these be used to create incoherence? I seem to recall hearing this once. I'm in general in favor of including these, but would want to reflect on the coherence question a bit.

OverloadedStrings: This seems very straightforward. But it's quite easy to conceive of times when I wouldn't want this feature in the language. If the extension becomes part of the standard, is there a way to turn it off? If not, Haskell2020 would not be a conservative extension of Haskell2010.

GADTs: While GADTSyntax seems straightforward, GADT type inference is far from. Do we have a description of GADT type inference that doesn't rely on thinking about variable unification? Even if we're OK putting unification into the standard, GHC's current inference capabilities are still not very finely mapped. (Note that the inference algorithm in the OutsideIn paper is incomplete with respect to the specification in that paper.)

TypeFamilies: I think there's quite a bit more to understand about type families before they could be standardized. First off, they come in many flavors (associated, open, closed). Do we want to include all of the flavors? How do partial type families really operate? I wrote a blog post [1] exploring this topic and had more questions than answers. What about termination checking? Or do we want to allow non-terminating definitions and discard decidability of type inference?

Of course, I recognize that John picked just a few arbitrary popular extensions. But even these extensions have a lot of dark corners, and my answers above are just to highlight that there's real work to be done, even with very popular extensions.

Richard

[1]: https://typesandkinds.wordpress.com/2015/09/09/what-are-type-families/

On May 2, 2016, at 6:57 PM, "John Wiegley" <johnw at newartisans.com> wrote:

> I wonder if there are GHC extensions we'd like to promote as features in the
> next report, as a starting point for discussing new additions.
> 
> There are a few GHC features that have become part of the regular Haskell
> landscape, such that it's hard to imagine a modern Haskell without them. For
> example, MultiParamTypeClasses, OverloadedStrings, GADTs, TypeFamilies, etc.
> 
> How much "work" is typically involved in promoting a feature to be in the
> Report, and how do we determine when it's a bad idea?
> 
> -- 
> John Wiegley                  GPG fingerprint = 4710 CF98 AF9B 327B B80F
> http://newartisans.com                          60E1 46C4 BD1A 7AC1 4BA2
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