Single-file Haskell scripts with Cabal directives

[chris-martin]

From the Cabal documentation:

Cabal also enables us to run single-file Haskell scripts without creating a project directory or .cabal file. The cabal directives are placed in the file within a comment.

#!/usr/bin/env cabal
{- cabal:
build-depends: base, split
-}

import Data.List.Split (chunksOf)

main :: IO ()
main = getLine >>= print . chunksOf 3

If I follow this example, I can indeed run this script at the command line. However, Haskell language server gives an error:

[{
	"resource": "[...].hs",
	"owner": "Haskell (invalid)",
	"severity": 8,
	"message": "Could not find module ‘Data.List.Split’\nIt is not a module in the current program, or in any known package.",
	"source": "not found",
	"startLineNumber": 6,
	"startColumn": 8,
	"endLineNumber": 6,
	"endColumn": 23
}]

Do I correctly infer that HLS does not support this feature? Are there any plans to?

[fendor]

Do I correctly infer that HLS does not support this feature?

Yes, that's correct.

Are there any plans to?

Of course, that'd be great. In theory, it is pretty simple to support, we just need a way to find out how to compile this file.

In an experiment, I found out that the following setup seems to be working, would you mind confirming?

Add a hie.yaml like this:

cradle:
  cabal:

and an empty cabal.project file.

Changes to the script part are not picked up, so you have to restart HLS in that case.

[chris-martin]

Can confirm, that does work! (GHC 9.2.8, cabal-install 3.10.1.0, haskell-language-server 2.0.0.0)