---

Index

• Description
  • 1. Development.GitRev
  • 2. Development.GitRev.Typed
    • Out-of-tree builds
  • 3. Development.GitRev.Typed.OsString
• Library Comparisons
  • Gitrev
  • Githash

Description

This is a fork of the popular gitrev package, offering Template Haskell splices for git revision information.

There are three interfaces:

1. Development.GitRev

This module provides the same interface as gitrev's Development.GitRev i.e. untyped splices:

-- Definition in Development.GitRev
gitHash :: ExpQ

{-# LANGUAGE TemplateHaskell #-}

import Development.GitRev qualified as GR

-- Returns a hash like "e67e943dd03744d3f93c21f84e127744e6a04543" or
-- "UNKNOWN", if something goes wrong.
myHash :: String
myHash = $(GR.gitHash)

2. Development.GitRev.Typed

This module -- on the other hand -- provides typed splices e.g.

-- Definition in Development.GitRev.Typed
gitHash :: Code Q String

{-# LANGUAGE TemplateHaskell #-}

import Development.GitRev.Typed qualified as GRT

-- Returns a hash like "e67e943dd03744d3f93c21f84e127744e6a04543" or
-- "UNKNOWN", if something goes wrong.
myHash :: String
myHash = $$(GRT.gitHash)

We also provide combinators for defining custom behavior. For instance, we can instead define a variant that fails at compile-time instead of returning the string UNKNOWN.

-- gitHashQ :: Q (Either GitError String)
-- projectError :: Q (Either e String) -> Q String
-- qToCode :: Q a -> Code Q a
myHashOrDie :: String
myHashOrDie = $$(GRT.qToCode $ GRT.projectError GRT.gitHashQ)

Out-of-tree builds

Furthermore, we have workarounds for "out-of-tree" builds:

{-# LANGUAGE OverloadedLists #-}

myHashEnv :: Code Q String
myHashEnv = toCode gitHash
  where
    toCode :: Q (Either (Exceptions GitRevError) String) -> Code Q String
    toCode = GRT.qToCode . GRT.projectError

    gitHash :: Q (Either (Exceptions GitRevError) String)
    gitHash =
      -- Tries, in order:
      --
      -- 1. Retrieving the git hash, as normal.
      -- 2. Running the git action under the directory pointed to by the
      --    environment variable EXAMPLE_HOME, if it exists.
      -- 3. Looking up environment variable EXAMPLE_HASH, returning the
      --    value if it exists.
      GRT.firstSuccessQ
        [ GRT.embedGitError GRT.gitHashQ,
          GRT.runGitInEnvDirQ "EXAMPLE_HOME" GRT.gitHashQ
          GRT.embedEnvError $ GRT.envValQ "EXAMPLE_HASH",
        ]

For example, myHashEnv will work for cabal install if we include the environment variable:

$ export EXAMPLE_HOME=$(pwd); cabal install example

This function will also work with nix flakes:

# flake.nix
let
  compiler = pkgs.haskell.packages."ghc9101";
in
{
  # Using nixpkgs haskell infra i.e. developPackage.
  packages.default = compiler.developPackage {
    name = "example";
    root = ./.;
    returnShellEnv = false;
    modifier =
      drv:
      let
        drv' = pkgs.haskell.lib.addBuildTools drv [
          compiler.cabal-install
          compiler.ghc
          pkgs.git
          pkgs.zlib
        ];
      in
      drv'.overrideAttrs (oldAttrs: {
        EXAMPLE_HASH = "${self.rev or self.dirtyRev}";
      });
  };
};

See the example in the repo for example usage, and Development.GitRev.Typed for full documentation.

3. Development.GitRev.Typed.OsString

Development.GitRev.Typed for OsString rather than String.

Library Comparisons

Overview

• 🌕: Supported.
• 🌓: Partial support.
• 🌑: Not supported.

👇 Feature / Library 👉	gitrev-typed	gitrev	githash
Maintained	🌕	🌑	🌓
Untyped TH	🌕	🌕	🌑
Typed TH	🌕	🌑	🌕
Custom git queries	🌕	🌑	🌑
"Out-of-tree" workarounds	🌕	🌑	🌑
OsString support	🌕	🌑	🌑

Gitrev

• As stated, this is a fork of gitrev. While gitrev has been admirably stable for many years, it was officially declared unmaintained in January 2024, and the repo was archived.

• Not only is gitrev-typed currently maintained, it offers the new, typed interface. Additionally, gitrev-typed resolves a number of open gitrev issues (see the changelog).

Githash

• Githash is another gitrev fork, though it takes a different approach. Rather than offering multiple, individual git queries, githash returns a single one with all of the (pre-defined) git data at once. The user can then take whatever subset they wish.

  This provides nice ergonomics when you want to run multiple queries at once, though it is arguably wasteful when you just want one or two. It also makes it harder to fit custom user queries into the API.

• Both githash and gitrev-typed have a typed TH interface (unlike gitrev).

• githash does not support custom git queries, workarounds for "out-of-tree" builds, or OsString.