Document current resolution of versions (string prefix)

See #248

setup/README.md

@@ -224,6 +224,15 @@ In contrast, a proper `boolean` input like `cabal-update` only accepts values `t
 
 ## Version Support
 
+This action is conscious about the tool versions specified in [`versions.json`](src/versions.json).
+This list is replicated (hopefully correctly) below.
+
+Versions specified by the inputs, e.g. `ghc-version`, are resolved against this list,
+by taking the first entry from the list if `latest` is requested,
+or the first entry that is a (string-)extension of the requested version otherwise.
+E.g., `8.10` will be resolved to `8.10.7`, and so will `8.10.`, `8.` and `8`
+(and incorrectly, [even `8.1`](github.com/haskell/actions/issues/248)).
+
 **GHC:**
 
 - `latest` (default)

setup/__tests__/find-haskell.test.ts

@@ -67,13 +67,13 @@ describe('haskell/actions/setup', () => {
   });
 
   it('Versions resolve correctly', () => {
-    const v = {ghc: '8.6.5', cabal: '2.4.1.0', stack: '2.1.3'};
+    const v = {ghc: '8.6.5', cabal: '3.4.1.0', stack: '1.9.3'};
     forAllOS(os => {
       const options = getOpts(def(os), os, {
         'enable-stack': 'true',
-        'stack-version': '2.1',
+        'stack-version': '1',
         'ghc-version': '8.6',
-        'cabal-version': '2.4'
+        'cabal-version': '3.4'
       });
       forAllTools(t => expect(options[t].resolved).toBe(v[t]));
     });
@@ -96,6 +96,19 @@ describe('haskell/actions/setup', () => {
     });
   });
 
+  it('Versions resolve as string prefix (resolving 8.1 to 8.10.x should be considered a bug)', () => {
+    const v = {ghc: '8.10.7', cabal: '2.4.1.0', stack: '2.1.3'};
+    forAllOS(os => {
+      const options = getOpts(def(os), os, {
+        'enable-stack': 'true',
+        'stack-version': '2.1',
+        'ghc-version': '8.1',
+        'cabal-version': '2'
+      });
+      forAllTools(t => expect(options[t].resolved).toBe(v[t]));
+    });
+  });
+
   it('Enabling stack does not disable GHC or Cabal', () => {
     forAllOS(os => {
       const {ghc, cabal, stack} = getOpts(def(os), os, {

setup/src/opts.ts

@@ -86,6 +86,7 @@ export function getDefaults(os: OS): Defaults {
 }
 
 // E.g. resolve ghc latest to 9.4.2
+// resolve ghc 8.1 to 8.10.7 (bug, https://github.com/haskell/actions/issues/248)
 function resolve(
   version: string,
   supported: string[],