Handling git repositories in projects

Both stack.yaml and cabal.project files can contain references to git repositories containing the version of a particular package that we wish to use. This is mostly handled automatically by nix-tools and haskell.nix however when we want to use a nix system that is configured to use restricted mode (typically hydra) it will need additional hashes for the referenced repositories.

When using project, cabalProject or stackProject functions you can include the hash needed in a comment.

To calculate the hash use nix-prefetch-git:

$ nix-prefetch-git https://github.com/input-output-hk/haskell.nix.git bc01ebc05a8105035c9449943046b46c8364b932
  "url": "https://github.com/input-output-hk/haskell.nix.git",
  "rev": "bc01ebc05a8105035c9449943046b46c8364b932",
  "date": "2019-05-30T13:13:18+08:00",
  "sha256": "003lm3pm024vhbfmii7xcdd9v2rczpflxf7gdl2pyxia7p014i8z",
  "fetchSubmodules": false


Add a --sha256 comment to the cabal.project file:

  type: git
  location: https://github.com/input-output-hk/haskell.nix.git
  tag: bc01ebc05a8105035c9449943046b46c8364b932
  subdir: test/cabal-simple
  --sha256: 003lm3pm024vhbfmii7xcdd9v2rczpflxf7gdl2pyxia7p014i8z


Add a # nix-sha256 comment to the stack.yaml file:

- git: https://github.com/input-output-hk/haskell.nix.git
  commit: bc01ebc05a8105035c9449943046b46c8364b932
    - test/cabal-simple
  # nix-sha256: 003lm3pm024vhbfmii7xcdd9v2rczpflxf7gdl2pyxia7p014i8z

Avoiding modifying cabal.project and stack.yaml

In some cases we cannot modify the cabal.project or stack.yaml file to add sha256 comments. As an alternative we can pass in a sha256map. For instance, pandoc includes a cabal.project file on hackage which includes a source-repository-package stanza for pandoc-citeproc:

{ haskell-nix, testSrc } :
  pandoc = haskell-nix.hackage-package {
    name         = "pandoc";
    version      = "";
    index-state  = "2020-04-15T00:00:00Z"; 
    # Function that returns a sha256 string by looking up the location
    # and tag in a nested attrset
    sha256map =
      { "https://github.com/jgm/pandoc-citeproc"."0.17"
          = "0dxx8cp2xndpw3jwiawch2dkrkp15mil7pyx7dvd810pwc22pm2q"; };