Expose cabal.project.freeze parsing in the Cabal library

[Fuuzetsu]

Currently it seems impossible to get at contents of cabal.project.freeze file without re-implementing a bunch of the Distribution.Client.Config that's only in the cabal-install binary and not available to be used.

Our specific use-case is that we generate stack.yaml file as a shim from our cabal files and the freeze file contains the set of flags that we should be using.

In general, it seems that parsing this file should be inside Cabal.

[phadej]

Asking to add cabal.project.freeze parsing to Cabal library is the same as adding parsing support for stack.yaml there.

In the current state of configuration code, splitting out the parser of cabal-install files (cabal.config, cabal.project), and having even some degree of compatibility is unfortunately impossible.

[Fuuzetsu]

Asking to add cabal.project.freeze parsing to Cabal library is the same as adding parsing support for stack.yaml there.

I'm not sure how this would be. We already do the parsing in cabal-install using the Cabal package, it seems like some of the code should just migrate back into Cabal...

In the current state of configuration code, splitting out the parser of cabal-install files (cabal.config, cabal.project), and having even some degree of compatibility is unfortunately impossible.

We already parse .cabal files, isn't it exactly the same issue? Is Cabal only meant to parse the .cabal file and offer tools to parse other things in the cabal format, but nothing specific to cabal-install? Hence the stack comment. If yes, feel free to close. It is very unfortunate though. Maybe cabal-install should expose modules so we could just invoke an already written piece of code from there...

---

In the meantime, if anyone is interested, I am parsing out the bits I'm interested in (package flags in constraints) with following code:

parseCabalFlags :: Action CabalFlags
parseCabalFlags = do
    need [cabalProjectFreezeFile]
    fields <- do
        freezeFileBs <- liftIO $ BS.readFile cabalProjectFreezeFile
        case Cabal.readFields freezeFileBs of
            Left e -> fail $ show e
            Right fields -> pure fields
    constraints <- case listToMaybe $ mapMaybe getConstraints fields of
        Nothing -> fail "No constraints section in cabal freeze file."
        Just constraints -> pure constraints

    let cabalFlags =
            let f m t = case Cabal.simpleParse (T.unpack t) of
                    Nothing -> m
                    Just (CabalPkgFlags pkg flags) ->
                        HashMap.insertWith (<>) pkg flags m
            in foldl' f mempty constraints
    pure $! CabalFlags cabalFlags
    where
        getConstraints (Cabal.Field (Cabal.Name _ n) ls)
            | n == fromString "constraints" = pure $ ls <&> \case
                Cabal.FieldLine _ t -> T.decodeUtf8 t
        getConstraints _ = Nothing

data CabalPkgFlags = CabalPkgFlags !T.Text !Cabal.FlagAssignment
instance Cabal.Parsec CabalPkgFlags where
    parsec = do
        -- We don't check the package name: if there are no flags on this
        -- line, parsecFlagAssignment will fail or be empty and we won't
        -- include this anyway.
        pkg <- Cabal.munch1 (not . isSpace)
        () <- Cabal.skipSpaces1
        flags <- Cabal.parsecFlagAssignment
        guard . not $ Cabal.nullFlagAssignment flags
        -- Consume any trailing spaces and trailing comma if it's not the last
        -- item.
        Cabal.spaces <* Cabal.optional (Cabal.char ',')
        pure $! CabalPkgFlags (fromString pkg) flags

[phadej]

Honestly, i don't think #7358 really closes this. Figuring out how to parse cabal.project(.freeze) using current cabal-install is beyond human. It's in theory possible, in practice it isn't.

[emilypi]

Was this closed by linking? I didn't physically close this issue. Thanks for reopening @phadej

[fgaz]

Yes, I thought I'd do good by linking the merge of that pr with the closure of the issues listed in "fixes", my bad

[emilypi]

No worries. I went through and this is the only one that is partially fixed but not fully.