Add with-compiler field for cabal scripts

[lukel97]

Adds a new data type and field grammar for parsing script metadata.
Now it is possible to have scripts like:

#!/usr/bin/env cabal
{- cabal:
build-depends: base
with-compiler: ghc-8.6.5
-}
main = putStrLn "hey"

Progresses towards #5698

---

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[phadej]

There's no EOL newline. Not biggie, but if you can configure your editor for this project, it would be nice next time!

[lukel97]

@phadej @DanielG ping

[hvr]

At this point, we should really start "versioning" the "script spec format" and make use of the version field right after the cabal: marker, e.g.

{- cabal: 3.2
build-depends: base ^>= { 4.12.0, 4.13.0 }
with-compiler: ghc-8.6.5
-}

[phadej]

I’d use `cabal-version: 3.2` everywhere. We have machinery for that, also finding static`cabal:` marker is easier. Motivation: less ad-hoc syntax/grammar, even it’s slightly more verbose

…

On 10 Nov 2019, at 10.34, Herbert Valerio Riedel ***@***.***> wrote: At this point, we should really start "versioning" the "script spec format" and make use of the version field right after the cabal: marker, e.g. {- cabal: 3.2 build-depends: base ^>= { 4.12.0, 4.13.0 } with-compiler: ghc-8.6.5 -} — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub, or unsubscribe.

[23Skidoo]

/cc @typedrat

[mpickering]

I would also like to be able to specify the compiler to use on the command line. For example

cabal File.hs -wghc-8.6.5

It seems the option is currently just ignored?

[hvr]

I would also like to be able to specify the compiler to use on the command line. For example

cabal File.hs -wghc-8.6.5

It seems the option is currently just ignored?

@mpickering It's not ignored; you're invoking the special shebang interface;

If you had used the normal designated CLI invocation for cabal scripts via cabal v2-run -wghc-8.6.5 File.hs you would have been fine.

Consider this simple script:

#! /usr/bin/env cabal

{- cabal:
build-depends: base
-}

import System.Environment

main = print =<< getArgs

Then, compare the outputs of the three invocations:

$ cabal ./foo.hs -wghc-7.4.2 -- --help

["-wghc-7.4.2","--","--help"]

$ cabal v2-run ./foo.hs -wghc-7.4.2 -- --help

["--help"]

$ ./foo.hs -wghc-7.4.2 -- --help

["-wghc-7.4.2","--","--help"]

Does it make sense now?

(NB: Only the one with v2-run is being explicitly compiled with ghc-7.4.2; the other two invocations pick up the default configured ghc version for compilation).

[mpickering]

Thanks. I did not know you could use v2-run with an executable script.

[typedrat]

LGTM!

[phadej]

I think @hvr raised the valid concern of need to versioning. I'll assign this to myself and merge after 3.0.1.0 and 3.2.0.0 dust is settled and add cabal versioning to this. If we version libraries, we definitely should version the tool that reads the definitions, as they will evolve (and possibly in non backward compatible ways).

[gbaz]

@phadej is this something you still intended to get to?

[gbaz]

is this superceded by #7997 ?

[bacchanalia]

With #7997 you can specify with-compiler inside the {- project: ... -} meta block, so I think it does supercede this.

[Mikolaj]

In that case, let me close.

@bubba: thank you for the PR and apologies that it dragged on for so long. If you think there's anything in the PR that can get rebased, scavenged and put to good use, please kindly open a new PR and we'll do our best not to lose it again.