Add (#<|) and deprecate its alias (<|#)

[wismill]

Previously the documentation stated that (#<|) was not supported by the parser. It turns out it is a silly interaction with -XUnboxedTuples. Adding extra spacing between the parentheses and the function fixes the issue.

This commit adds (#<|) and makes (<|#) its deprecated alias.

[wismill]

Note: fourmolu does not support this fix and suppresses the spaces in ( #<| ). See tweag/ormolu#1062.

For now, I use fourmolu’s special comments to disable formatting at the corresponding lines.

[Bodigrim]

Thanks, interesting!

Could you please mention the caveat about spaces around ( #<| ) in documentation? Lest users are tricked the same way I was.

I'm a bit reluctant to accumulate pragmas and would rather wait for Ormolu / Formolu to fix the formatting issue.

[wismill]

Could you please mention the caveat about spaces around ( #<| ) in documentation?

I added a mention about the caveat in the doc and in the code.

Lest users are tricked the same way I was.

I discovered this caveat by mere curiosity: GHC had no issue with a #<| b but had with (#<|). So it is indeed not obvious!

I'm a bit reluctant to accumulate pragmas and would rather wait for Ormolu / Formolu to fix the formatting issue.

Agreed. I reduced to only one comment at the beginning of the file. A fix is already in the work, see tweag/ormolu#1063. I do not think you should wait for a formatting tool to be fixed to merge this PR, as it introduces a deprecation in a young library: the sooner you release, the lesser users will need migration.

[wismill]

Rebased. I tested with success formatting with fourmolu using the patch at tweag/ormolu#1063.

[Bodigrim]

Waiting for fourmolu/fourmolu#369 to be released.

[wismill]

Fourmolu 0.14 has been released

[wismill]

I removed the commit with the special Fourmolu comments.