We should use separate failures for trailing spaces and for line length. How are we going to know which one failed?

This is only processing *.hs files in the current directory. There are no such files in the repo root. So it would always succeed.

Also, can we do the line-length processing on git diff output so that we do not have to worry about fixing the existing long lines.

This won't work. The output will always be empty as there is nothing changed in the checked out tree. You need to do git diff between the current branch HEAD and the target branch in which the PR is to be merged. The current branch and target branch info should be available in some env vars.

Also you could just directly pipe the output of diff to the command below rather than putting it in a file first.

In github, ${{ github.event.pull_request.head.sha }} is the pull request base commit.

git diff ${{ github.event.pull_request.head.sha }} HEAD 

will work for all pull requests as expected.

In circleci, I am unable to immediately find the env var that has the branch or the commit being compared to.
Some interesting env vars are,

• CIRCLE_BRANCH: Current branch name
• CIRCLE_SHA1: Current checkout latest hash

A trivial way if the merge branch is always origin/master is:

git diff origin/master HEAD 

You can probably use the github CI.

git diff ${{ github.event.pull_request.head.sha }} HEAD

should work as expected.

dit diff might produce lines that might be over 80 that we have no control of.
These might be the git info messages, separators, etc.
We need to filter only lines that start with a +.
We can probably solve the whole task with sed.

The git diff output has deleted lines, changed lines and other info. We need to grep only the lines with a leading + (grep '^+') and filter out the lines with a leading +++(grep -v^+++`) from that.

Ah, yes!