#+TITLE: Hacking

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* Basic workflow

Entering the development shell using ~nix-shell~ will pull
in all the dependencies required by all sub-packages.

Both ~default.nix~ and ~shell.nix~ simply import ~<nixpkgs>~ from your
~NIX_PATH~. If this is by chance broken, try using ~nixpkgs-unstable~ which
is targeted by our CI, as it is also the basis for ~haskell-updates~ ~nixpkgs~ branch
where new Hackage packages land.

Not pinning the to specific version of ~nixpkgs~ is intentional so CI can roll
with the ~unstable~ branch and detect breakage early.

** Using another version of GHC

To use a different version of GHC to the one used
by your ~pkgs.haskellPackages~, pass, for example ~--argstr compiler ghc963~
to either ~nix-shell~ or ~nix-build~

** ~matrix.nix~

~nix-build matrix-nix~ can be used to check that all our packages
built against all supported GHC versions. It pulls the versions
from [[../.github/workflows/ci.dhall][.github/workflows/ci.dhall]] so there is a single source of truth
and the compilers don't need to be listed separately again.

* Adding new packages

When adding a new sub-package, following files need to be updated:
+ ~default.nix~
+ ~shell.nix~
+ ~cabal.project~
+ ~cabal.project.local.ci~ (only if needed)
+ ~hie.yaml~

* CI

The GitHub Actions CI uses [[https://github.com/sorki/github-actions-dhall][github-actions-dhall]] and [[../.github/workflows/ci.dhall][.github/workflows/ci.dhall]]
to generate the [[../.github/workflows/ci.yaml][.github/workflows/ci.yaml]] workflow file
which does a full matrix build of supported compilers both using ~cabal~ and ~nix~.

** Cachix cache

The CI uses [[https://github.com/cachix/cachix/][cachix]] to not rebuild everything from scratch everytime it runs, it also
feeds the cache available at https://app.cachix.org/cache/hnix-store which you
can configure on your system.

** Updating

To update the CI, edit [[../.github/workflows/ci.dhall][.github/workflows/ci.dhall]] and run [[../.github/workflows/ci.sh][.github/workflows/ci.sh]]
to regenerate the ~yaml~ file. Don't forget to commit both files.

** ~cabal.project.local.ci~

The [[../cabal.project.local.ci][cabal.project.local.ci]] file is used by the CI workflow to alter ~ghc-options~,
enabling e.g. ~-Werror~ and ~-Wunused-packages~. You can copy this to ~cabal.project.local~
to have the same build configuration:

#+begin_src shell
cp cabal.project.local.ci cabal.project.local
#+end_src

* Changelogs

Since the packages are used by others as dependencies in production environments,
make sure to add changelog entries for public interface breaking changes. These don't
need to cover all changes if you are sure that the change only affects packages inside
~hnix-store~.

* Readmes

Some ~README.md~ files are symlinked to ~README.lhs~ files and also serve as a buildable executables.
This makes sure they are always up to date and buildable. They are guarded by cabal flags
and only available in development environment which enables all of them via [[../cabal.project][cabal.project]], which
also causes them to be built by CI but not to propagate downstream (and polute ~$PATH~ of downstream users).

* ~ghcid~

~ghcid~ can be used as a lightweight IDE to assist with edit-compile-run-test cycle. You can obtain it quickly using
~nix-shell -p haskellPackages.ghcid~

** Running
*** For a single library

#+begin_src shell
ghcid -c 'cabal repl hnix-store-core'
#+end_src

*** For a testsuite or executable

#+begin_src shell
ghcid -c 'cabal repl test-suite:props'
# or
ghcid -c 'cabal repl exe:db-readme'
#+end_src

Often the specifier like ~exe~ or ~test-suite~ can be omitted when the name is unique.

*** Running a testsuite after a build

If working with the testsuite directly, you can invoke
#+begin_src shell
ghcid -c 'cabal repl test-suite:remote' --test 'hspec spec'
#+end_src

If you are editing a library or you need to run multiple testsuites, you can for example use

#+begin_src shell
ghcid -c 'cabal repl hnix-store-remote' --test ':! cabal test test-suite:remote && cabal test test-suite:remote-io'
#+end_src

*** With restarting

~ghcid~ can also restart itself so ~ghci~ picks up new or moved files, following incantation can be invoked
when working with ~hnix-store-remote~ to combine all of the features

#+begin_src shell
ghcid -c 'cabal repl hnix-store-remote' \
  --restart 'hnix-store-remote/hnix-store-remote.cabal' \
  --test ':! cabal test test-suite:remote && cabal test test-suite:remote-io'
#+end_src