data-files-with-backpack.md

There'd be a standard signature somewhere, which I will call Dist.DataFiles, and it'd look like this

module Dist.DataFiles(getDataFileName) where
  getDataFileName :: FilePath -> IO FilePath

This would have a handful of basic, built-in implementations, including the often-standard

module Dist.DataFiles.Here(getDataFileName) where
  getDataFileName = return

When I create a library that uses data files, I define it as depending on Data.DistFiles(getDataFileName), which is just a signature and doesn't have a concrete implementation

module Sample.Library (getFile) where

import Dist.DataFiles (getDataFileName)

getFile :: IO String
getFile = getDataFileName "sample" >>= readFile

Correspondingly, the .cabal file for this mentions that it's indefinite and needs an implementation of Dist.DataFiles to use:

name:                sample-library
indefinite:          True
build-depends:       base
exposed-modules:     Sample.Library
required-signatures: Dist.DataFiles

I can later on write an application that uses this library:

module Main where

import Sample.Library

main = getFile >>= putStrLn

In order to compile this, I need to tell sample-library which implementation of Dist.DataFiles to use. For example, I can tell it to use the naïve implementation above by writing a cabal file like this:

name: sample-executable
build-depends: base,
               sample-library (Dist.DataFiles.Here as Dist.DataFiles)

If I did not provide an implementation of Dist.DataFiles for sample-library, then this would not compile, because sample-library would still be indefinite.