cabal format adds empty lines with trailing whitespace

[tibbe]

If you run cabal format on e.g. in the cabal-install/ directory, you'll see that cabal format adds empty lines with trailing whitespace (i.e. 4 spaces). It shouldn't do that.

[ddssff]

I think this might be the same as #1921

[23Skidoo]

The Text.PrettyPrint module may be the cause of the problem - it doesn't do a very good job of avoiding trailing whitespace. Which can be annoying - one example is cabal init.

[osa1]

Cabal file generators(cabal format, cabal init, probably some other commands?) always do this. As also pointed out by @23Skidoo this is related with pretty library, which generates trailing space if we have a code like nest 4 (text "" $+$ ...).

I can think several solutions:

1. We can modify pretty library and add a new EmptyLine constructor to Doc, then handle that in vertical composition functions specially to not generate any whitespace, only generate a "\n". Then we can use emptyLine instead of text "".
2. We can make significant refactoring in Cabal pretty printer to not have any text "" inside nest. (this is probably very hard to do, and resulting code may not be very good)
3. We can wrap writeCabalFile and writeGenericPackageDescription with a function that removes trailing white space with the cost of an extra copy.

(3) seems to be the easiest solution, however I think (1) is the best, because as far as I can understand from the API docs of pretty, it doesn't provide a way to insert newlines, without having them indented and this seems to be a limitation of the library.

What do you think? Should we go with (3) for now?

[23Skidoo]

I like option (1) best, (3) could be a temporary workaround.

[osa1]

I implemented (1), but I need some reviewers. I'm trying to get in contact with pretty maintainers, hopefully we can fix this in a principled way.

[23Skidoo]

@osa1 Great, thank you for working on this.

[byorgey]

What's the status of this? @osa1 , can you post here a link to your implementation of (1)?

[osa1]

I feel like it's trivial to implement for someone who knows internals of pretty, and I asked for help here haskell/pretty#26 because I don't know the internals, and lack of documentation is a problem. Main problem is that even though it's possible to do without adding any new Doc constructor, some internals, documentation and maybe even invariants need to be changed, and it's not trivial for me to figure out what to change. I think I can spare some time for this after tomorrow.

[gbaz]

Is this resolved?

[phadej]

Not yet. #5737 resolve this though.

[jneira]

#5737 is merged so I am gonna close it, @tibbe feel free to reopen if it is not resolved for you