Implement parse-cabal-file

.github/workflows/test.yml

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+name: parse-cabal-file
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      -
+        uses: actions/checkout@v3
+      -
+        run: ghc -Wall -Werror ParseCabalFile.hs
+
+  test:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os:
+          - ubuntu-latest
+          - ubuntu-22.04
+          - ubuntu-20.04
+          - ubuntu-18.04
+          - macos-latest
+          - macos-12
+          - macos-11
+          - windows-latest
+          - windows-2022
+          - windows-2019
+    defaults:
+      run:
+        shell: bash
+    steps:
+      -
+        uses: actions/checkout@v3
+      -
+        name: Write test cabal file
+        run: |
+          echo 'name: foo' >> foo.cabal
+          echo 'version: 12.34' >> foo.cabal
+      -
+        name: Run action
+        uses: ./
+        id: cabal_file
+        with:
+          cabal_file: foo.cabal
+      -
+        name: Check action worked
+        run: |
+          if [[ "${{ steps.cabal_file.outputs.version }}" != "12.34" ]]; then
+            echo 'Action failed!' >&2
+            exit 1
+          fi
+
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      -
+        uses: actions/checkout@v3
+      -
+        uses: fourmolu/fourmolu-action@v7
+        with:
+          pattern: '*.hs'
+          extra-args: >
+            --indentation 2
+            --indent-wheres true

ParseCabalFile.hs

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+{-# LANGUAGE ImportQualifiedPost #-}
+{-# LANGUAGE LambdaCase #-}
+
+import Data.ByteString qualified as BS
+import Data.List (intercalate)
+import Distribution.Package (packageVersion)
+import Distribution.PackageDescription.Parsec (parseGenericPackageDescriptionMaybe)
+import Distribution.Version (versionNumbers)
+import System.Environment (getArgs)
+import System.IO (hPutStrLn, stderr, stdout)
+
+main :: IO ()
+main = do
+  cabalFile <-
+    getArgs >>= \case
+      [cabalFile] -> return cabalFile
+      _ -> errorWithoutStackTrace "Expected exactly one argument: CABAL_FILE"
+
+  cabalFileContents <- BS.readFile cabalFile
+  packageDesc <-
+    maybe (errorWithoutStackTrace "Could not parse cabal file") return $
+      parseGenericPackageDescriptionMaybe cabalFileContents
+
+  output
+    [ ("version", intercalate "." . map show . versionNumbers . packageVersion $ packageDesc)
+    ]
+
+output :: [(String, String)] -> IO ()
+output = mapM_ (uncurry output')
+  where
+    output' key value = do
+      let kv = key ++ "=" ++ value
+      -- log to stderr
+      hPutStrLn stderr $ "Setting: " ++ kv
+      -- output to stdout into $GITHUB_OUTPUT
+      hPutStrLn stdout kv

README.md

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+# `haskell-actions/parse-cabal-file`
+
+GitHub Action: Parse Cabal file
+
+Assumes the runner has GHC installed.
+
+## Inputs
+
+* `cabal_file` (required): The path to a cabal file
+
+## Outputs
+
+* `version`: The version in the Cabal file
+
+## Example
+
+```yaml
+jobs:
+  my-job:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+
+    - uses: haskell-actions/parse-cabal-file@v1
+      id: cabal_file
+      with:
+        cabal_file: my-library.cabal
+
+    - run: echo ${{ steps.cabal_file.outputs.version }}
+```

action.yml

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+name: Parse Cabal File
+description: Parse information in a provided cabal file
+
+inputs:
+  cabal_file:
+    description: Path to cabal file
+    required: true
+
+outputs:
+  version:
+    description: The version in the cabal file
+    value: ${{ steps.cabal_file.outputs.version }}
+
+runs:
+  using: composite
+  steps:
+    - run: |
+        BINDIR=${GITHUB_ACTION_PATH}/bin
+        mkdir -p ${BINDIR}
+        ghc ${GITHUB_ACTION_PATH}/ParseCabalFile.hs -outputdir /tmp -o ${BINDIR}/parse-cabal-file
+        ${BINDIR}/parse-cabal-file "${{ inputs.cabal_file }}" >> "${GITHUB_OUTPUT}"
+      id: cabal_file
+      shell: bash