libnixt: allow eval decoded expression (#388)

libnixt/meson.build

@@ -40,7 +40,12 @@ libnixt_test_exe = executable(
     dependencies: [ gtest_main, nixt ]
 )
 
-subdir('tools')
+nixt_decode = executable(
+    'nixt-decode',
+    'tools/nixt-decode.cpp',
+    dependencies: [ nixt ],
+    install: true
+)
 
 test(
     'unit/libnixt',

libnixt/tools/nixt-decode/Main.cpp → libnixt/tools/nixt-decode.cpp

@@ -3,7 +3,24 @@
 #include "nixt/InitEval.h"
 #include "nixt/PtrPool.h"
 
-int main() {
+namespace {
+
+// --eval, -e
+// Should we perform eval?
+bool Eval = false;
+
+void parseArgs(int Argc, const char *Argv[]) {
+  for (int I = 0; I < Argc; I++) {
+    std::string_view Arg(Argv[I]);
+    if (Arg == "--eval" || Arg == "-e")
+      Eval = true;
+  }
+}
+
+} // namespace
+
+int main(int Argc, const char *Argv[]) {
+  parseArgs(Argc, Argv);
   nixt::initEval();
   std::unique_ptr<nix::EvalState> State(
       new nix::EvalState{{}, nix::openStore("dummy://")});
@@ -24,7 +41,18 @@ int main() {
 
   auto *AST = nixt::deserializeHookable(Data, Ctx, Pool, VMap, EMap);
 
-  AST->show(State->symbols, std::cout);
+  if (Eval) {
+    try {
+      AST->bindVars(*State, State->staticBaseEnv);
+      nix::Value V;
+      State->eval(AST, V);
+      V.print(State->symbols, std::cout);
+    } catch (nix::BaseError &E) {
+      std::cerr << E.what() << "\n";
+    }
+  } else {
+    AST->show(State->symbols, std::cout);
+  }
 
   return 0;
 }