`shelly` unbuildable with GHC 8.0.2+

Question

`shelly` unbuildable with GHC 8.0.2+

Closed this issue 8 years ago · 9 comments

It turns out that GHC 8.0.2 is a bit more strict re Haskell2010, and now rejects a default (Text) declaration more eagerly (rather than lazily as it did before):

src/Shelly/Pipe.hs:122:1-16: error:
    • The default type ‘Text’ is not an instance of ‘Num’
    • When checking the types in a default declaration

There's two ways to fix this in src/Shelly/Pipe.hs, by either

removing the redundant default (Text) declaration, or
adding a {-# LANGUAGE ExtendedDefaultRules #-} pragma

Answer 1 · 2016-11-26T15:58:23.000Z

I just noticed this too. (It might be nice to make a note of this in the GHC 8.0.2 release notes...)

On a related topic, do you know why the error message mentions Num? Or why enabling ExtendedDefaultRules fixes it?

Answer 2 · 2016-11-30T00:32:29.000Z

For the record, Ryan's note will be present in the 8.0.2 release notes.

Answer 3 · 2016-11-30T00:36:12.000Z

For the record, Ryan's note will be present in the 8.0.2 release notes.

@bgamari: Wait, what? :)

I don't think we've addressed this particular regression yet—primarily because I don't even understand what's changed between 8.0.1 and 8.0.2 to make this issue appear. It would be mighty helpful if someone answer a subset of these questions:

Why do we need to enable ExtendedDefaultRules in 8.0.2 but not 8.0.1 (and moreover, if you can point to a GHC commit in which this behavior changed)?
Why is the error message is mentioning Num in the first place?

Admittedly, this appears to be a very much a corner case, as this is the only package where I encountered this particular class of error. But it'd be nice to document nonetheless.

Answer 4 · 2016-11-30T01:03:56.000Z

Oops! I mixed up this with GHC #12881. Sorry about that!

Answer 5 · 2016-12-01T01:52:29.000Z

I figured out the commit that caused this - see GHC Trac #12907.

Answer 6 · 2016-12-01T08:05:33.000Z

@RyanGlScott as I explained in the GHC ticket, the particular code GHC refuses to compile is in fact illegal under (plain) Haskell2010.

Answer 7 · 2017-01-07T07:14:11.000Z

Fwiw, #137 would be nice to have for GHC 8.0.2 as well (but it's not as critical)

Answer 8 · 2017-02-27T10:40:19.000Z

Any chance of a release fixing this ? darcsden test suite uses shelly.

Answer 9 · 2017-03-03T20:38:51.000Z

It looks to me like today's shelly-1.6.8.2 and shelly-1.6.8.3 releases (which btw are identical, except for the version number) address the problem at hand, so I'm considering this ticket closed as far as I'm concerned.