flake improvements

[disassembler]

• exposes project from haskell.nix at top level
• adds work-around so --update-input with CHaP works as expected

Changelog

- description: |
    Flake improvements:
    - exposes project from haskell.nix at top level
    - adds work-around so --update-input with CHaP works as expected
  compatibility: no-api-changes
  type: maintenance

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• The change log section in the PR description has been filled in
• New tests are added if needed and existing tests are updated. These may include:
  • golden tests
  • property tests
  • roundtrip tests
• Any changes are noted in the CHANGELOG.md for affected package
• The version bounds in .cabal files are updated
• CI passes. See note on CI. The following CI checks are required:
  • Code is linted with hlint. See .github/workflows/check-hlint.yml to get the hlint version
  • Code is formatted with stylish-haskell. See .github/workflows/stylish-haskell.yml to get the stylish-haskell version
  • Code builds on Linux, MacOS and Windows for ghc-8.10.7 and ghc-9.2.7
• Self-reviewed the diff

Note on CI

If your PR is from a fork, the necessary CI jobs won't trigger automatically for security reasons.
You will need to get someone with write privileges. Please contact IOG node developers to do this
for you.

[angerman]

LGTM

Although the CHaP workaround could be better in explaining why this is necessary right now.

[newhoggy]

Merging, but @disassembler can you please add an explanation as suggested by @angerman to the PR?

[hamishmack]

From #69:

This helps haskell.nix find the package source correctly. In particular new packages that are not in the main CHaP branch yet will be loaded correctly.

To try it out you will need a local CHaP, this can be build with:

git clone git@github.com:input-output-hk/cardano-haskell-packages.git
cd cardano-haskell-packages
nix develop -c foliage build -j 0 --write-metadata

That will make a cardano-haskell-packages/_repo dir with the CHaP repository including any new packages. To use from a directory containing this cardano-api branch run:

nix build --override-input CHaP path:/home/hamish/iohk/cardano-haskell-packages/_repo .\#hydraJobs.required

Make sure to use path: and the full path to the _repo directory.

[hamishmack]

Also we can update this once input-output-hk/haskell.nix#1984 is merged.