All you wanted to know about GHCup development.

Module graph

Module graph

Main functionality is in GHCup module. Utility functions are organised tree-ish in GHCup.Utils and GHCup.Utils.*.

Anything dealing with ghcup specific directories is in GHCup.Utils.Dirs.

Download information on where to fetch bindists from is in the appropriate yaml files: data/metadata/ghcup-<yaml-ver>.yaml.

Design decisions

Using Excepts as a beefed up ExceptT

This is an open variant, similar to plucky or oops and allows us to combine different error types. Maybe it is too much and it's a little bit unergonomic at times. If it really hurts maintenance, it will be removed. It was more of an experiment.

No use of haskell-TLS

I consider haskell-TLS an interesting experiment, but not a battle-tested and peer-reviewed crypto implementation. There is little to no research about what the intricacies of using haskell for low-level crypto are and how vulnerable such binaries are. Instead, we use either curl the binary or wget. There's also an implementation based on OpenSSL bindings, but it isn't enabled by default, since it would complicate shipping static binaries.

Optics instead of lens

They're a little safer (less Monoid weirdness with view) and have better error messages. Consider the following with lens:

> view (_Just . to (++ "abc")) Nothing

vs optics:

> view (_Just % to (++ "abc")) Nothing

<interactive>:2:1: error:
    • An_AffineFold cannot be used as A_Getter
    • In the expression: view (_Just % to (++ "abc")) Nothing
      In an equation for ‘it’: it = view (_Just % to (++ "abc")) Nothing

StrictData on by default

Kazu Yamamoto explained it in his PR very well. I like to agree with him. The instances where we need non-strict behavior, we annotate it.

Strict is a little more odd as a default, since it depends on how you define your functions as well.

Code style and formatting

Unfortunately, code formatters are semi-broken on this codebase, due to TH and CPP.

Some light suggestions:

  1. mtl-style preferred
  2. no overly pointfree style
  3. use where a lot, so the main function body reads like prose
  4. documentation is part of the code

Common Tasks

Adding a new GHC version

  1. open the latest data/metadata/ghcup-<yaml-ver>.yaml
  2. find the latest ghc version (in yaml tree e.g. ghcupDownloads -> GHC -> 8.10.7)
  3. copy-paste it
  4. adjust the version, tags, changelog, source url
  5. adjust the various bindist urls (make sure to also change the yaml anchors)
  6. run cabal run exe:ghcup-gen -- check -f data/metadata/ghcup-<yaml-ver>.yaml
  7. run cabal run exe:ghcup-gen -- check-tarballs -f data/metadata/ghcup-<yaml-ver>.yaml -u 'ghc-8\.10\.8'

Adding a new CLI command

An example illustration on how to deal with optparse-applicative can be seen here:

Major refactors

  1. First major refactor included adding cross support. This added GHCTargetVersion, which includes the target in addition to the version. Most of the Version parameters to functions had to be replaced with that and ensured the logic is consistent for cross and non-cross installs.
  2. This refactor added windows support wrt #130. The major changes here were switching hpath library out for filepath/directory (sadly) and introducing a non-unix way of handling processes via the process library. It also introduced considerable amounts of CPP wrt file handling, installation etc.


  1. Update version in ghcup.cabal and boostrap-haskell (ghver variable at the top of the script)

  2. Update GHCup.Version module. ghcupURL must only be updated if we change the GHCupInfo type or the YAML representation of it. The version of the YAML represents the change increments. ghcUpVer is the current application version, read from ghcup.cabal.

  3. Add ChangeLog entry

  4. Add/fix downloads in ghcup-<ver>.yaml (under data/metadata), then verify with ghcup-gen check -f data/metadata/ghcup-<ver>.yaml and possibly (example only) ghcup-gen check-tarballs -f data/metadata/ghcup-<ver>.yaml -u 'ghc-8.10.7'. Generally, new GHC/cabal/stack/hls versions are only added to the latest yaml file. New GHCup versions are added to all (great care must be taken here to not break the parser... e.g. ARM platforms don't parse in all older formats).

  5. Commit and git push with tag. Wait for tests to succeed and release artifacts to build.

  6. Download release artifacts and upload them along with checksum files (sha256sum --tag * > SHA256SUMS && gpg --detach-sign -u <your-email> SHA256SUMS)

  7. Add ghcup release artifacts to ALL yaml files (see point 4.)

  8. Upload the final data/metadata/ghcup-<ver>.yaml (and a detached GPG sig of it) to

  9. Update bootstrap-haskell and bootstrap-haskell.ps1 to

  10. Update the top-level ghcup symlinks at

  11. Post on reddit/discourse/etc. and collect rewards


This documentation page is built via mkdocs, see mkdocs.yml and docs/ subfolder. The module graph needs graphmod and is generated via scripts/dev/