Show format for --help flag. Fixes #2460

[psibi]

Fixes #2460

[23Skidoo]

It's intentionally hidden because it's somewhat buggy.

[gbaz]

Now that #2353 is closed, are there any other blockers on exposing this?

[bgamari]

@23Skidoo?

[23Skidoo]

@bgamari Sorry, I'm sick right now, will answer in a few days.

[bgamari]

@23Skidoo ping (apologies if you are still ill).

[dcoutts]

Patch looks fine, assuming we're now happy with exposing this feature.

[23Skidoo]

Well, the patch itself is trivial, but format has been a constant source of bugs, so I'd like to see some more testing done before merging. One suggestion is to go through all .cabal files on Hackage and check that readPackageDescription path is equivalent to (readPackageDescription path >>= writeGenericPackageDescription path) >> readPackageDescription path. Looking through open tickets I see #2661, maybe there is more. Other enhancements that'd be nice to see: #1966, #1127.

[phadej]

Simple check for @23Skidoo read . write . read = read, using https://github.com/commercialhaskell/all-cabal-files

module Main (main) where

import Control.Monad
import Control.Applicative
import Distribution.Verbosity
import Distribution.PackageDescription.Parse
import Distribution.PackageDescription.PrettyPrint
import Distribution.ParseUtils

hoist :: ParseResult a -> Maybe a
hoist (ParseOk _ x) = Just x
hoist (ParseFailed _) = Nothing

check :: FilePath -> IO ()
check cabal = do
  print cabal
  desc <- readPackageDescription silent cabal
  let desc' = parsePackageDescription (showGenericPackageDescription desc)
  when (Just desc /= hoist desc') $ do
    print $ Just desc
    print $ hoist desc'
    fail cabal

main :: IO ()
main = do
  -- Generate listing with
  -- find . -type f -name '*.cabal' > cabal-files
  cabalFiles <- lines <$> readFile "cabal-files"
  mapM_ check cabalFiles

Fails already at ./accelerate-cuda/0.12.0.0/accelerate-cuda.cabal on my machine (using Cabal in master)

[23Skidoo]

@phadej Thanks! I think we shouldn't expose format until this test passes for all .cabal files on Hackage.

[23Skidoo]

Current status: blocked by #2661 and #2681.

[BardurArantsson]

I'm sure this is nice'n'all, but I'm not sure "round-tripping" is the be-all-and-end-all of package description formats[1]. So, I'd argue to just say WONTFIX... as unpopular as that may be. I'd rather argue that the cmdline arg and all supporting code should be removed.

(I hope I haven't completely misunderstood the point of this PR.)

[1] Parsers of said formats, sure, but not the semantics.

[m-renaud]

I have recently started taking a closer look at the cabal format command and there are quite a few changes that we need to make before this is generally available. I have started tracking the work at https://github.com/haskell/cabal/projects/9, expect some updates there in the coming weeks.

I'm going to close out this PR since it'll be a little while before we want this to be published.