Use ghcup to install cabal head (#290)

This allows `head` to be the specified cabal version.

Commits:
* Use ghcup to install cabal head
* Add cabal head to cabal-version description
* Add cabal head test and check
* Add cabal head to README supported versions
* Use wildcard to check that version is prefix
* Make sed version grabber cross platform

.github/workflows/workflow.yml

@@ -48,6 +48,9 @@ jobs:
           # Latest releases
           - ghc:   latest
             cabal: latest
+          # Latest ghc with cabal head
+          - ghc:   latest
+            cabal: head
           # Recommended releases (update according to ghcup)
           # 9.4 will be recommended soon
           - ghc:   "9.4"
@@ -177,7 +180,17 @@ jobs:
           else
             GHCVER_EXPECTED="${{ steps.setup.outputs.ghc-version }}"
           fi
-          [[ "${CABALVER}" == "${{ steps.setup.outputs.cabal-version }}" ]] && \
+          if [[ "${{ steps.setup.outputs.cabal-version }}" == "head" ]]
+          then
+            CABALVER_EXPECTED=$( \
+              curl --silent https://raw.githubusercontent.com/haskell/cabal/master/cabal-install/cabal-install.cabal | \
+              sed -E -n 's/^Version:[[:space:]]+//p' \
+            )
+            echo "Cabal head: ${CABALVER_EXPECTED}"
+          else
+            CABALVER_EXPECTED="${{ steps.setup.outputs.cabal-version }}"
+          fi
+          [[ "${CABALVER_EXPECTED}" == ${CABALVER}* ]] && \
             [[ "${GHCVER}" == "${GHCVER_EXPECTED}" ]]
 
       - name: Test runghc

README.md

@@ -286,6 +286,7 @@ Suggestion: Try to support at least the three latest major versions of GHC.
 
 **Cabal:**
 
+- `head` (the [cabal-head](https://github.com/haskell/cabal/releases/tag/cabal-head) release of the most recent build of the `master` branch)
 - `latest` (default, recommended)
 - `3.10.1.0` `3.10`
 - `3.8.1.0` `3.8`

action.yml

@@ -8,7 +8,7 @@ inputs:
     default: 'latest'
   cabal-version:
     required: false
-    description: 'Version of Cabal to use. If set to "latest", it will always get the latest stable version.'
+    description: 'Version of Cabal to use. If set to "latest", it will always get the latest stable version. If set to "head", it will always get the latest build of cabal.'
     default: 'latest'
   stack-version:
     required: false

src/installer.ts

@@ -346,8 +346,36 @@ export async function addGhcupReleaseChannel(
 async function ghcup(tool: Tool, version: string, os: OS): Promise<void> {
   core.info(`Attempting to install ${tool} ${version} using ghcup`);
   const bin = await ghcupBin(os);
-  const returnCode = await exec(bin, ['install', tool, version]);
-  if (returnCode === 0) await exec(bin, ['set', tool, version]);
+  if (tool === 'cabal' && version === 'head') {
+    await ghcupCabalHead(os, bin);
+  } else {
+    const returnCode = await exec(bin, ['install', tool, version]);
+    if (returnCode === 0) await exec(bin, ['set', tool, version]);
+  }
+}
+
+function cabalHeadUrlTag(os: OS): string {
+  switch (os) {
+    case 'linux':
+      return 'Linux';
+    case 'darwin':
+      return 'macOS';
+    case 'win32':
+      return 'Windows';
+  }
+}
+
+async function ghcupCabalHead(os: OS, bin: string): Promise<void> {
+  const osTag = cabalHeadUrlTag(os);
+  const cabalHeadUrl = `https://github.com/haskell/cabal/releases/download/cabal-head/cabal-head-${osTag}-x86_64.tar.gz`;
+  const returnCode = await exec(bin, [
+    'install',
+    'cabal',
+    '-u',
+    cabalHeadUrl,
+    'head'
+  ]);
+  if (returnCode === 0) await exec(bin, ['set', 'cabal', 'head']);
 }
 
 async function ghcupGHCHead(): Promise<void> {