Allow primops to have Markdown documentation

doc/manual/src/expressions/builtins.md

@@ -9,9 +9,6 @@ scope. Instead, you can access them through the `builtins` built-in
 value, which is a set that contains all built-in functions and values.
 For instance, `derivation` is also available as `builtins.derivation`.
 
-  - `abort` *s*; `builtins.abort` *s*  
-    Abort Nix expression evaluation, print error message *s*.
-
   - `builtins.add` *e1* *e2*  
     Return the sum of the numbers *e1* and *e2*.
 

src/libexpr/eval.cc

@@ -525,6 +525,34 @@ Value * EvalState::addPrimOp(const string & name,
 }
 
 
+Value * EvalState::addPrimOp(PrimOp && primOp)
+{
+    /* Hack to make constants lazy: turn them into a application of
+       the primop to a dummy value. */
+    if (primOp.arity == 0) {
+        primOp.arity = 1;
+        auto vPrimOp = allocValue();
+        vPrimOp->type = tPrimOp;
+        vPrimOp->primOp = new PrimOp(std::move(primOp));
+        Value v;
+        mkApp(v, *vPrimOp, *vPrimOp);
+        return addConstant(primOp.name, v);
+    }
+
+    Symbol envName = primOp.name;
+    if (hasPrefix(primOp.name, "__"))
+        primOp.name = symbols.create(std::string(primOp.name, 2));
+
+    Value * v = allocValue();
+    v->type = tPrimOp;
+    v->primOp = new PrimOp(std::move(primOp));
+    staticBaseEnv.vars[envName] = baseEnvDispl;
+    baseEnv.values[baseEnvDispl++] = v;
+    baseEnv.values[0]->attrs->push_back(Attr(primOp.name, v));
+    return v;
+}
+
+
 Value & EvalState::getBuiltin(const string & name)
 {
     return *baseEnv.values[0]->attrs->find(symbols.create(name))->value;

src/libexpr/eval.hh

@@ -30,6 +30,8 @@ struct PrimOp
     PrimOpFun fun;
     size_t arity;
     Symbol name;
+    std::vector<std::string> args;
+    const char * doc = nullptr;
 };
 
 
@@ -240,6 +242,8 @@ private:
     Value * addPrimOp(const string & name,
         size_t arity, PrimOpFun primOp);
 
+    Value * addPrimOp(PrimOp && primOp);
+
 public:
 
     Value & getBuiltin(const string & name);

src/libexpr/primops.cc

@@ -445,12 +445,19 @@ static void prim_genericClosure(EvalState & state, const Pos & pos, Value * * ar
 }
 
 
-static void prim_abort(EvalState & state, const Pos & pos, Value * * args, Value & v)
+static RegisterPrimOp primop_abort({
+    .name = "abort",
+    .args = {"s"},
+    .doc = R"(
+      Abort Nix expression evaluation and print the error message *s*.
+    )",
+    .fun = [](EvalState & state, const Pos & pos, Value * * args, Value & v)
     {
         PathSet context;
         string s = state.coerceToString(pos, *args[0], context);
         throw Abort("evaluation aborted with the following error message: '%1%'", s);
     }
+});
 
 
 static void prim_throw(EvalState & state, const Pos & pos, Value * * args, Value & v)
@@ -2238,7 +2245,20 @@ RegisterPrimOp::RegisterPrimOp(std::string name, size_t arity, PrimOpFun fun,
     std::optional<std::string> requiredFeature)
 {
     if (!primOps) primOps = new PrimOps;
-    primOps->push_back({name, arity, fun, requiredFeature});
+    primOps->push_back({
+        .name = name,
+        .args = {},
+        .arity = arity,
+        .requiredFeature = std::move(requiredFeature),
+        .fun = fun
+    });
+}
+
+
+RegisterPrimOp::RegisterPrimOp(Info && info)
+{
+    if (!primOps) primOps = new PrimOps;
+    primOps->push_back(std::move(info));
 }
 
 
@@ -2314,7 +2334,6 @@ void EvalState::createBaseEnv()
     addPrimOp("__isBool", 1, prim_isBool);
     addPrimOp("__isPath", 1, prim_isPath);
     addPrimOp("__genericClosure", 1, prim_genericClosure);
-    addPrimOp("abort", 1, prim_abort);
     addPrimOp("__addErrorContext", 2, prim_addErrorContext);
     addPrimOp("__tryEval", 1, prim_tryEval);
     addPrimOp("__getEnv", 1, prim_getEnv);
@@ -2431,7 +2450,13 @@ void EvalState::createBaseEnv()
     if (RegisterPrimOp::primOps)
         for (auto & primOp : *RegisterPrimOp::primOps)
             if (!primOp.requiredFeature || settings.isExperimentalFeatureEnabled(*primOp.requiredFeature))
-                addPrimOp(primOp.name, primOp.arity, primOp.primOp);
+                addPrimOp({
+                    .fun = primOp.fun,
+                    .arity = std::max(primOp.args.size(), primOp.arity),
+                    .name = symbols.create(primOp.name),
+                    .args = std::move(primOp.args),
+                    .doc = primOp.doc,
+                });
 
     /* Now that we've added all primops, sort the `builtins' set,
        because attribute lookups expect it to be sorted. */

src/libexpr/primops.hh

@@ -10,9 +10,11 @@ struct RegisterPrimOp
     struct Info
     {
         std::string name;
-        size_t arity;
-        PrimOpFun primOp;
+        std::vector<std::string> args;
+        size_t arity = 0;
+        const char * doc;
         std::optional<std::string> requiredFeature;
+        PrimOpFun fun;
     };
 
     typedef std::vector<Info> PrimOps;
@@ -26,6 +28,8 @@ struct RegisterPrimOp
         size_t arity,
         PrimOpFun fun,
         std::optional<std::string> requiredFeature = {});
+
+    RegisterPrimOp(Info && info);
 };
 
 /* These primops are disabled without enableNativeCode, but plugins

src/nix/repl.cc

@@ -416,7 +416,8 @@ bool NixRepl::processLine(string line)
              << "  :r            Reload all files\n"
              << "  :s <expr>     Build dependencies of derivation, then start nix-shell\n"
              << "  :t <expr>     Describe result of evaluation\n"
-             << "  :u <expr>     Build derivation, then start nix-shell\n";
+             << "  :u <expr>     Build derivation, then start nix-shell\n"
+             << "  :doc <expr>   Show documentation of a builtin function\n";
     }
 
     else if (command == ":a" || command == ":add") {
@@ -509,6 +510,24 @@ bool NixRepl::processLine(string line)
     else if (command == ":q" || command == ":quit")
         return false;
 
+    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) {
+                // FIXME: format markdown.
+                if (!v2->primOp->args.empty())
+                    std::cout << fmt("Arguments: %s\n\n", concatStringsSep(" ", v2->primOp->args));
+                std::cout << trim(stripIndentation(v2->primOp->doc)) << "\n";
+            } else
+                throw Error("builtin function '%s' does not have documentation", v2->primOp->name);
+        } else
+            throw Error("value does not have documentation");
+    }
+
     else if (command != "")
         throw Error("unknown command '%1%'", command);