Support inspecting mutable variables

.github/workflows/debugger.yaml

@@ -56,12 +56,12 @@ jobs:
     continue-on-error: true # Run all jobs in the matrix to the end
     strategy:
       matrix:
-        version: [9.14.0.20250908] #, latest-nightly] disable nightly while its broken
+        version: [9.14.0.20251007] #, latest-nightly] disable nightly while its broken
         include:
           # - channel: https://ghc.gitlab.haskell.org/ghcup-metadata/ghcup-nightlies-0.0.7.yaml
           #   version: latest-nightly
           - channel: https://raw.githubusercontent.com/haskell/ghcup-metadata/refs/heads/develop/ghcup-prereleases-0.0.9.yaml
-            version: 9.14.0.20250908
+            version: 9.14.0.20251007
     steps:
       - uses: actions/checkout@v4
         with:
@@ -110,12 +110,12 @@ jobs:
     continue-on-error: true # Run all jobs in the matrix to the end
     strategy:
       matrix:
-        version: [9.14.0.20250908] #, latest-nightly] disable nightly while its broken
+        version: [9.14.0.20251007] #, latest-nightly] disable nightly while its broken
         include:
           # - channel: https://ghc.gitlab.haskell.org/ghcup-metadata/ghcup-nightlies-0.0.7.yaml
           #   version: latest-nightly
           - channel: https://raw.githubusercontent.com/haskell/ghcup-metadata/refs/heads/develop/ghcup-prereleases-0.0.9.yaml
-            version: 9.14.0.20250908
+            version: 9.14.0.20251007
     steps:
       - uses: actions/checkout@v4
         with:

.github/workflows/release.yaml

@@ -30,7 +30,7 @@ jobs:
 
       - uses: haskell-actions/setup@v2
         with:
-          ghc-version: 9.14.0.20250908
+          ghc-version: 9.14.0.20251007
           cabal-version: 3.14
           ghcup-release-channel: https://raw.githubusercontent.com/haskell/ghcup-metadata/refs/heads/develop/ghcup-prereleases-0.0.9.yaml
 

haskell-debugger.cabal

@@ -2,7 +2,7 @@ cabal-version:      3.12
 name:               haskell-debugger
 version:            0.9.0.0
 synopsis:
-    A step-through machine-interface debugger for GHC Haskell
+    A step-through debugger for GHC Haskell
 
 description:        This package provides a standalone executable
                     called @hdb@ which can be used to step-through Haskell

haskell-debugger/GHC/Debugger/Runtime.hs

@@ -49,6 +49,8 @@ obtainTerm key = do
                     Nothing -> error "Couldn't find labeled field in dataConFieldLabels"
               NewtypeWrap{wrapped_term} ->
                 wrapped_term -- regardless of PathFragment
+              RefWrap{wrapped_term} ->
+                wrapped_term -- regardless of PathFragment
               _ -> error "Unexpected term for the given TermKey"
        in do
         term <- getTerm key

haskell-debugger/GHC/Debugger/Stopped/Variables.hs

@@ -78,6 +78,10 @@ termVarFields top_key top_term =
             wvi <- obtainTerm key >>= termToVarInfo key
             return (LabeledFields [wvi])
           _ -> error "unexpected number of Newtype fields: larger than 1"
+      RefWrap{wrapped_term=_{- don't use directly! go through @obtainTerm@ -}} -> do
+        let key = FromPath top_key (PositionalIndex 1)
+        wvi <- obtainTerm key >>= termToVarInfo key
+        return (IndexedFields [wvi])
       _ -> return NoFields
 
 

test/integration-tests/data/T92/T92.hs

@@ -0,0 +1,10 @@
+module Main where
+
+import Data.IORef
+
+main = do
+  r <- newIORef False
+  putStrLn (const "hi" r) -- make r free in this line
+  writeIORef r True
+  putStrLn (const "bye" r) -- what does r look like now?
+

test/integration-tests/test/adapter.test.ts

@@ -565,11 +565,53 @@ describe("Debug Adapter Tests", function () {
             const _1_1_1_1_1_1Var = await _1_1_1_1_1Child.get('_1');
             assertIsString(_1_1_1_1_1_1Var, '"hello"');
         })
+
+        it('inspect mutable variables (#92)', async () => {
+            let config = mkConfig({
+                  projectRoot: "/data/T92",
+                  entryFile: "T92.hs",
+                  entryPoint: "main",
+                  entryArgs: [],
+                  extraGhcArgs: []
+                })
+
+            const expected = { path: config.projectRoot + "/" + config.entryFile, line: 7 }
+            await dc.hitBreakpoint(config, { path: config.entryFile, line: 7 }, expected, expected);
+
+            async function getMutVarValue() {
+                let locals = await fetchLocalVars();
+                const rVar = await locals.get('r');
+                assert.strictEqual(rVar.value, "IORef (STRef (GHC.Prim.MutVar# _))")
+                const rChild = await expandVar(rVar);
+                const r_1_Var = await rChild.get('_1'); // No force
+                assert.strictEqual(r_1_Var.value, 'STRef');
+                const r_1_Child = await expandVar(r_1_Var);
+                const r_1_1_Var = await r_1_Child.get("_1"); // No force
+                assert.strictEqual(r_1_1_Var.value, 'GHC.Prim.MutVar# _');
+                const r_1_1_Child = await expandVar(r_1_1_Var);
+                const r_1_1_1_Var = await forceLazy(r_1_1_Child.get("_1")); // FORCE REFERENCE!
+                return r_1_1_1_Var
+            }
+
+            const m1 = await getMutVarValue()
+            assert.strictEqual(m1.value, "False")
+            await dc.nextRequest({ threadId: 0 });
+            await dc.nextRequest({ threadId: 0 });
+            await dc.nextRequest({ threadId: 0 });
+            // Now we're at the start of the last line, where the ref should be True
+            // Note how we get it from scratch, the content of the ref must be
+            // forced again (the forceLazy call in getMutVarValue)
+            const m2 = await getMutVarValue()
+            assert.strictEqual(m2.value, "True")
+        })
     })
     describe("Stepping out (step-out)", function () {
 
-        let step_out_broken = ghc_version < "9.15.20250731" // hasn't been merged yet, but let's use this bound; will probably only be in GHC 9.14.2
-        let need_opt = step_out_broken
+        // TODO: Add simpler tests which don't rely on optimisations at all.
+        // E.g. just simply stepping out to a case expression
+
+        let step_out_broken = ghc_version < "9.14.0.20251007" // hasn't been merged yet, but let's use this bound; will probably only be in GHC 9.14.2
+        let need_opt = true // Currently we depend on this to work around the fact that >>= is in library code because base is not being interpreted
 
         // Mimics GHC's T26042b
         it('without tail calls', async () => {
@@ -596,7 +638,7 @@ describe("Debug Adapter Tests", function () {
 
             // back to main
             await dc.stepOutRequest({threadId: 0});
-            await dc.assertStoppedLocation('step', expected(step_out_broken ? 6 : 6));
+            await dc.assertStoppedLocation('step', expected(step_out_broken ? 6 : 5));
 
             // exit
             await dc.stepOutRequest({threadId: 0});