Add a REPL command if supported

NixOS/nix#3934 allows for custom REPL commands
to be added. This had my name on it in several regards ;-)

default.nix

@@ -12,14 +12,18 @@ let
           cargo = self.latest.rustChannels.stable.rust;
         }
       )
+      # You can put an overlay for your dev version of nix here.
+      # This is not great UX. I am sorry. I am not sure how to do it better :(
+      # Maybe flakes.
+      # (import ../nix).overlay
     ];
   };
   inherit (import sources.gitignore { inherit (pkgs) lib; }) gitignoreSource;
   naersk = pkgs.callPackage sources.naersk {};
 in
 naersk.buildPackage {
   name   = "nix-doc";
-  version = "0.3.3";
+  version = "0.4.0";
 
   src = gitignoreSource ./.;
 

nix-doc/Cargo.toml

@@ -3,7 +3,7 @@ authors = ["lf- <[email protected]>"]
 description = "Nix documentation grepping tool"
 edition = "2018"
 name = "nix-doc"
-version = "0.3.3"
+version = "0.4.0"
 license = "LGPL-3.0-or-later"
 homepage = "https://github.com/lf-/nix-doc"
 repository = "https://github.com/lf-/nix-doc"

plugin/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "nix-doc-plugin"
-version = "0.1.1"
+version = "0.2.0"
 edition = "2018"
 
 [lib]

plugin/build.rs

@@ -42,14 +42,23 @@ fn main() {
         .probe("nix-main")
         .unwrap();
 
-    cc::Build::new()
+    let nix_ver = nix_expr.version.clone();
+
+    let mut build = cc::Build::new();
+    build
         .cpp(true)
         .opt_level(2)
         .shared_flag(true)
         .flag("-std=c++17")
         .add_pkg_config(nix_expr)
         .add_pkg_config(nix_store)
         .add_pkg_config(nix_main)
-        .file("plugin.cpp")
-        .compile("nix_doc_plugin.so");
+        .file("plugin.cpp");
+
+    // Indicate that we need to patch around an API change with macros
+    if nix_ver.chars().take(1).next().unwrap() >= '3' {
+        build.define("NIX_3_0_0", None);
+    }
+
+    build.compile("nix_doc_plugin.so");
 }

plugin/plugin.cpp

@@ -12,6 +12,11 @@
 
 #endif
 
+#ifdef NIX_3_0_0
+#define throwTypeError_ throwTypeError
+#define throwTypeError(msg, val, pos) throwTypeError_(pos, msg, val)
+#endif
+
 using namespace nix;
 
 extern "C" {
@@ -40,37 +45,66 @@ void prim_getDoc(EvalState & state, const nix::Pos & pos, Value * * args, Value
     }
 }
 
-/* Return documentation of the given lambda. */
-void prim_printDoc(EvalState & state, const nix::Pos & pos, Value * * args, Value & v)
+void printLambdaDocs(Value & v)
 {
-    /* ensure the argument is a function */
-    state.forceValue(*args[0], pos);
-    if (args[0]->type != tLambda) {
-        throwTypeError("%2%: value is %1% while a lambda was expected", *args[0], pos);
-    }
-
-    auto poz = args[0]->lambda.fun->pos;
+    auto poz = v.lambda.fun->pos;
     std::string const & file = poz.file;
     char const * doc = nd_get_function_docs(file.c_str(), poz.line, poz.column);
-    mkNull(v);
     if (doc != nullptr) {
         std::cout << doc << std::endl;
         nd_free_string(doc);
     }
+
+}
+
+void forceLambda(Value & v, const Pos & pos)
+{
+    if (v.type != tLambda) {
+        throwTypeError("%2%: value is %1% while a lambda was expected", v, pos);
+    }
+}
+
+/* Return documentation of the given lambda. */
+void prim_printDoc(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    /* ensure the argument is a function */
+    state.forceValue(*args[0], pos);
+    forceLambda(*args[0], pos);
+
+    printLambdaDocs(*args[0]);
+    mkNull(v);
 }
 
 /* Return position information of the given lambda. */
 void prim_unsafeGetLambdaPos(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     /* ensure the argument is a function */
     state.forceValue(*args[0], pos);
-    if (args[0]->type != tLambda) {
-        throwTypeError("%2%: value is %1% while a lambda was expected", *args[0], pos);
-    }
+    forceLambda(*args[0], pos);
 
     state.mkPos(v, &args[0]->lambda.fun->pos);
 }
 
 static RegisterPrimOp rp1("__getDoc", 1, prim_getDoc);
 static RegisterPrimOp rp2("__doc", 1, prim_printDoc);
 static RegisterPrimOp rp3("__unsafeGetLambdaPos", 1, prim_unsafeGetLambdaPos);
+
+// only include the :doc repl command if our nix supports it
+// https://github.com/NixOS/nix/pull/3934
+#if __has_include("repl.hh")
+#include "repl.hh"
+
+void replCmd(NixRepl & repl, string, string arg)
+{
+    Value v;
+    repl.evalString(arg, v);
+    // it has been evaluated now
+    // TODO: (pending nix-doc hacking), support non-function attributes
+    // We actually have all the info to use unsafeGetAttrPos (more or less) with
+    // nice syntax, but I don't think nix-doc can get docs for attributes
+    forceLambda(v, noPos);
+    printLambdaDocs(v);
+}
+
+static RegisterReplCmd rc1({"doc"}, "Get the `nix-doc` documentation for <expr>", replCmd, "<expr>");
+#endif