diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index a5ebd1bf0..4f2fd36c1 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -563,6 +563,25 @@ Value & EvalState::getBuiltin(const string & name)
 }
 
 
+std::optional<EvalState::Doc> EvalState::getDoc(Value & v)
+{
+    if (v.type == tPrimOp || v.type == tPrimOpApp) {
+        auto v2 = &v;
+        while (v2->type == tPrimOpApp)
+            v2 = v2->primOpApp.left;
+        if (v2->primOp->doc)
+            return Doc {
+                .pos = noPos,
+                .name = v2->primOp->name,
+                .arity = v2->primOp->arity,
+                .args = v2->primOp->args,
+                .doc = v2->primOp->doc,
+            };
+    }
+    return {};
+}
+
+
 /* Every "format" object (even temporary) takes up a few hundred bytes
    of stack space, which is a real killer in the recursive
    evaluator.  So here are some helper functions for throwing
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index db8eb3e16..80078d8a5 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -248,6 +248,17 @@ public:
 
     Value & getBuiltin(const string & name);
 
+    struct Doc
+    {
+        Pos pos;
+        std::optional<Symbol> name;
+        size_t arity;
+        std::vector<std::string> args;
+        const char * doc;
+    };
+
+    std::optional<Doc> getDoc(Value & v);
+
 private:
 
     inline Value * lookupVar(Env * env, const ExprVar & var, bool noEval);
diff --git a/src/nix/repl.cc b/src/nix/repl.cc
index c7f8af742..8ecf11900 100644
--- a/src/nix/repl.cc
+++ b/src/nix/repl.cc
@@ -514,23 +514,22 @@ bool NixRepl::processLine(string line)
     else if (command == ":doc") {
         Value v;
         evalString(arg, v);
-        if (v.type == tPrimOp || v.type == tPrimOpApp) {
-            auto v2 = &v;
-            while (v2->type == tPrimOpApp)
-                v2 = v2->primOpApp.left;
-            if (v2->primOp->doc) {
-                auto args = v2->primOp->args;
+        if (auto doc = state->getDoc(v)) {
+            std::string markdown;
+
+            if (!doc->args.empty() && doc->name) {
+                auto args = doc->args;
                 for (auto & arg : args)
                     arg = "*" + arg + "*";
 
-                auto markdown =
-                    "**Synopsis:** `builtins." + (std::string) v2->primOp->name + "` "
-                    + concatStringsSep(" ", args) + "\n\n"
-                    + trim(stripIndentation(v2->primOp->doc));
+                markdown +=
+                    "**Synopsis:** `builtins." + (std::string) (*doc->name) + "` "
+                    + concatStringsSep(" ", args) + "\n\n";
+            }
 
-                std::cout << renderMarkdownToTerminal(markdown);
-            } else
-                throw Error("builtin function '%s' does not have documentation", v2->primOp->name);
+            markdown += trim(stripIndentation(doc->doc));
+
+            std::cout << renderMarkdownToTerminal(markdown);
         } else
             throw Error("value does not have documentation");
     }