Merge pull request #11304 from hercules-ci/repl-doc-functor

`:doc`: support `__functor`

src/libexpr/eval.cc

@@ -616,6 +616,25 @@ std::optional<EvalState::Doc> EvalState::getDoc(Value & v)
                 strdup(ss.data()),
         };
     }
+    if (isFunctor(v)) {
+        try {
+            Value & functor = *v.attrs()->find(sFunctor)->value;
+            Value * vp = &v;
+            Value partiallyApplied;
+            // The first paramater is not user-provided, and may be
+            // handled by code that is opaque to the user, like lib.const = x: y: y;
+            // So preferably we show docs that are relevant to the
+            // "partially applied" function returned by e.g. `const`.
+            // We apply the first argument:
+            callFunction(functor, 1, &vp, partiallyApplied, noPos);
+            auto _level = addCallDepth(noPos);
+            return getDoc(partiallyApplied);
+        }
+        catch (Error & e) {
+            e.addTrace(nullptr, "while partially calling '%1%' to retrieve documentation", "__functor");
+            throw;
+        }
+    }
     return {};
 }
 
@@ -1471,26 +1490,9 @@ void ExprLambda::eval(EvalState & state, Env & env, Value & v)
     v.mkLambda(&env, this);
 }
 
-namespace {
-/** Increments a count on construction and decrements on destruction.
- */
-class CallDepth {
-  size_t & count;
-public:
-  CallDepth(size_t & count) : count(count) {
-    ++count;
-  }
-  ~CallDepth() {
-    --count;
-  }
-};
-};
-
 void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value & vRes, const PosIdx pos)
 {
-    if (callDepth > settings.maxCallDepth)
-        error<EvalError>("stack overflow; max-call-depth exceeded").atPos(pos).debugThrow();
-    CallDepth _level(callDepth);
+    auto _level = addCallDepth(pos);
 
     auto trace = settings.traceFunctionCalls
         ? std::make_unique<FunctionCallTrace>(positions[pos])