strong-path types are restricted to type parameters with kind *

[craigmc08]

Currently, the types in this package do not have kind polymorphism, meaning you can only have Dir a if a :: *. But sometimes it would be convenient to allow a to have a different kind.

For example, imagine I want to define a function readREADME :: Project -> IO String which gets the README file from a couple of explicitly defined projects:

data Project = WaspcProject | WasplsProject

To do this with strong-path, I would like to be able to write:

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE PolyKinds #-}

data Project = WaspcProject | WasplsProject

readREADME :: Project -> IO String
readREADME project =
  let readmeFile = case
        WaspcProject -> waspcProjectDir </> readmeFileInProjectDir
        WasplsProject -> wasplsProjectDir </> readmeFileInProjectDir
  in readFile $ toFilePath readmeFile

data READMEFile

readmeFileInProjectDir :: forall (base :: Project). Path' (Rel base) (File READMEFile)
readmeFileInProjectDir = [relfile|README.md|]

waspcProjectDir :: Path' Abs (Dir 'WaspcProject)
waspcProjectDir = [absdir|~/waspc|]

wasplsProjectDir :: Path' Abs (Dir 'WasplsProject)
wasplsProjectDir = [absdir|~/waspls|]

Currently, I have define some extra types WaspcDir and WasplsDir, and then I have to make sure to keep those types in-sync with Project in case I add more constructors. I would also have to make a typeclass to act similar to the Project kind that I could use to limit what readmeFileInProjectDir can be joined with. The goal is to reduce that little bit of boilerplate and work required to keep the constructors and types in-sync.

I think this should be possible to do by enabling -XPolyKinds in this package.

[Martinsos]

Oh interesting!

Package is relatively simple and has instructions for running and testing. If you want to give it a try and see if you can easily get it working, feel free to try to create a PR.
I will probably not do it very soon because I don't know anything about PolyKinds and only little about DataKinds, so it will take me some time to get to this.

[Martinsos]

Just to be clear: you want this in the example below so you don't have to write:

readmeFileInWaspcProjectDir :: Path' (Rel 'WaspcProject) (File READMEFile)
readmeFileInWaspcProjectDir = [relfile|README.md|]

readmeFileInWasplsProjectDir :: Path' (Rel 'WasplsProject) (File READMEFile)
readmeFileInWasplsProjectDir = [relfile|README.md|]

Instead, you want to capture it in a simpler fashion that both of these dirs have the README in same path inside them?

[craigmc08]

Just to be clear: you want this in the example below so you don't have to write:

readmeFileInWaspcProjectDir :: Path' (Rel 'WaspcProject) (File READMEFile)
readmeFileInWaspcProjectDir = [relfile|README.md|]

readmeFileInWasplsProjectDir :: Path' (Rel 'WasplsProject) (File READMEFile)
readmeFileInWasplsProjectDir = [relfile|README.md|]

Instead, you want to capture it in a simpler fashion that both of these dirs have the README in same path inside them?

I want this feature so I don't have to write:

data Project = WaspcProject | WasplsProject

readREADME :: Project -> IO String
readREADME project =
  let readmeFile = case
        WaspcProject -> waspcProjectDir </> readmeFileInProjectDir
        WasplsProject -> wasplsProjectDir </> readmeFileInProjectDir
  in readFile $ toFilePath readmeFile

data READMEFile

-- Duplication here: If I change definition of 'Project' I have to make sure to do the same thing to these types too
class ProjectDir where
data WaspcProjectDir deriving (ProjectDir)
data WasplsProjectDir deriving (ProjectDir)

readmeFileInProjectDir :: ProjectDir dir => Path' (Rel dir) (File READMEFile)
readmeFileInProjectDir = [relfile|README.md|]

waspcProjectDir :: Path' Abs (Dir WaspcProjectDir)
waspcProjectDir = [absdir|~/waspc|]

wasplsProjectDir :: Path' Abs (Dir WasplsProjectDir)
wasplsProjectDir = [absdir|~/waspls|]

[Martinsos]

Right, sure! That is one solution, with a class. Other solution is what I provided above, with repeating the path.