Pre commit git hook interferes with partially-staged files

[florentc]

We provide a pre-commit hook to automatically format source files with ormolu in ci/ormolu-pre-commit-hook.sh

The script adds (git add) the source files. When trying to only stage parts of a modified file for a commit (using git add -p for example), this hook overrides it and stages all the changes in the file, making it impossible to only commit some parts.

I don't think there is an easy way to sort this out. In theory we should be able to identify staged hunks and only apply ormolu on those lines, then stage these hunks again, but this seems far from trivial and this would be extremely low priority work.

Alternatives would be:

• Warning about the issue explicitly
• Prevent committing non well formatted files but leave the actual action to the user. Having guards to guarantee we always commit well formatted files is important for group work, but so are meaningful commits which may require partial staging. (Clément suggested we take a look at https://gitlab.com/tezos/tezos/-/blob/master/scripts/pre_commit/pre_commit.py)
• Issue an error if there is need to format some files but instead on relying on the user to do it, stash the changes so that they can be brought up and staged individually. (Claudio suggested that approach and the https://pre-commit.com/ framework)

[gabrielhdt]

The following hook would just prevent people from committing if some file is not properly formatted

#!/bin/sh
ormolu --mode check $(find . -name '*.hs')

[Niols]

In general, I am very much against commit hooks that do anything else that accepting/rejecting commits, so I'd be more for a solutoin like @gabrielhdt's. However, I would appreciate more granularity and I'd like to avoid my commits being rejected because of a file that is unstaged (or even ignored!). I have been using one quite successfully before, I'll find it back for us and we can see if you like it. Its only downside is that it rejects commits that stage only part of a file but where the unstaged part is badly formatted.

[gabrielhdt]

Maybe something like

#!/bin/sh
ormolu --mode check $(git diff --name-only --cached | grep '\.hs$')

which should only list files in the next commit

[Niols]

I remember trying something like this and being unsatisfied. I think it behaves poorly with added or deleted files? I ended up using git status --porcelain followed by sed for filtering and processing.