Multi module script support

[harendra-kumar]

Describe the bug
shebang script interpreter fails if the script spans over multiple modules

To Reproduce
ghc compiles this program successfully.

$ cat OtherModule.hs
module OtherModule (hello) where

hello :: String
hello = "hello"

$ cat hello.hs
#! /usr/bin/env cabal
{- cabal:
    build-depends: base
    ghc-options: -Wall
-}
module Main where

import Example.Hello (hello)

main = putStrLn hello

$ ghc hello.hs
[1 of 2] Compiling OtherModule      ( OtherModule.hs, OtherModule.o )
Linking hello ...

But cabal run fails:

$ cabal v2-run hello.hs
Resolving dependencies...
Build profile: -w ghc-8.8.3 -O1
In order, the following will be built (use -v for more details):
 - fake-package-0 (exe:script) (first run)
Configuring executable 'script' for fake-package-0..
Preprocessing executable 'script' for fake-package-0..
Building executable 'script' for fake-package-0..
[1 of 1] Compiling Main             ( Main.hs, /var/folders/p4/fdt36vy95f52t_3dnpcx8_340000gn/T/cabal-repl.-24197/dist-newstyle/build/x86_64-osx/ghc-8.8.3/fake-package-0/x/script/build/script/script-tmp/Main.o )

Main.hs:7:1: error:
    Could not find module ‘OtherModule’
    Use -v (or `:set -v` in ghci) to see a list of the files searched for.
  |
7 | import OtherModule (hello)
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^

Expected behavior
cabal script interpreter should be able to compile and run the above program.

System information

• Mac OSX

$ cabal --version
cabal-install version 3.2.0.0
compiled using version 3.2.0.0 of the Cabal library 

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.8.3

[phadej]

I think this is out of scope of interpreter functionality. At that point you should create a package.

cc @hvr

[harendra-kumar]

That's fine too, it will be nice if we can mention this in the user guide. I was just wondering if this is easy to implement.

[ju1m]

This would enable the usage of TemplateHaskell which can only run a function at compile time if it is imported from another module.
A workaround is to add hs-source-dirs: /absolute/path/to/directory, but this has poor portability.

[georgefst]

A workaround is to add hs-source-dirs: /absolute/path/to/directory

With 3.6.2.0, that's given me:

Warning: script block:5:57:
unexpected absolute FilePath
cabal: Failed parsing "script block".

And presumably relative paths is yet another thing blocked on #6977.

[bacchanalia]

A trick to fake an absolute path as a relative path is to add a while lot of ../../../../ to the front (enough to ensure you get all the way to root no mater where you want to use it from). I've been using this on my work for repl support baring a better solution being found so I'd expect it to work here too.

[bacchanalia]

I have multi module script support working in #7851

[fgaz]

Hmm, even though that commit makes it possible, I'd still not advertise multi-module scripts as supported. I agree with @phadej that at that point you really should create a package since you already have multiple files anyway

[jneira]

Also agree, you aways can create a dummy script to access the package like we did in hls: https://github.com/haskell/haskell-language-server/blob/master/install.hs

[bacchanalia]

Unfortunately, the final version of #7851 will not allow for multi-module scripts, because I had to balance it against other considerations. I think it's unlikely to be added in the future do to those same considerations.