Does not use apt-get installed GHCs

[sol]

Here is a repro:

jobs:
  build:
    runs-on: ubuntu-18.04
    steps:
      - uses: actions/checkout@v2
      - uses: haskell/actions/setup@v1
        with:
          cabal-version: '3.4'
          ghc-version: '7.4.1'
      - run: ghc --version

• installs 7.4.1: https://github.com/sol/haskell-setup-action-bug/runs/4094359642?check_suite_focus=true#step:3:88
• but uses 9.2.1: https://github.com/sol/haskell-setup-action-bug/runs/4094359642?check_suite_focus=true#step:4:4

[hazelweakly]

Oh this one is fun. Here's what's happening.

1. First the action installs GHC (from the ppa) and adds it to the path.
2. Then it installs cabal from ghcup, and adds it to the path.

The path looks like your-cabal-location:your-ghc-location:$PATH. But the problem is that ghcup installs cabal by default to ~/.ghcup/bin which contains... preinstalled GHCs for github actions. So ~/.ghcup/bin/ghc is v9.2.1 because that's what the github action runners installed last.

(This didn't come up before because almost always cabal and ghc come from the same source). I'll have to see if I can figure out an order-independent way to resolve this. I think the solution is going to be (for now) that I unset a few known things, but I'll have to be careful to not break stuff that's manually/pre-installed :)

[ncaq]

I had the same problem too.
My solution was to run the following command before setup.

- run: sudo rm -rf ~/.ghcup/bin/