1 files changed, 56 insertions, 0 deletions
diff --git a/third_party/nix/src/nix/eval.cc b/third_party/nix/src/nix/eval.cc
new file mode 100644
index 000000000000..72fcbd82717d
--- /dev/null
+++ b/third_party/nix/src/nix/eval.cc
@@ -0,0 +1,56 @@
+#include "libexpr/eval.hh"
+
+#include "libexpr/value-to-json.hh"
+#include "libmain/common-args.hh"
+#include "libmain/shared.hh"
+#include "libstore/store-api.hh"
+#include "libutil/json.hh"
+#include "nix/command.hh"
+
+namespace nix {
+struct CmdEval final : MixJSON, InstallableCommand {
+  bool raw = false;
+
+  CmdEval() { mkFlag(0, "raw", "print strings unquoted", &raw); }
+
+  std::string name() override { return "eval"; }
+
+  std::string description() override { return "evaluate a Nix expression"; }
+
+  Examples examples() override {
+    return {
+        Example{"To evaluate a Nix expression given on the command line:",
+                "nix eval '(1 + 2)'"},
+        Example{"To evaluate a Nix expression from a file or URI:",
+                "nix eval -f channel:nixos-17.09 hello.name"},
+        Example{"To get the current version of Nixpkgs:",
+                "nix eval --raw nixpkgs.lib.nixpkgsVersion"},
+        Example{"To print the store path of the Hello package:",
+                "nix eval --raw nixpkgs.hello"},
+    };
+  }
+
+  void run(ref<Store> store) override {
+    if (raw && json) {
+      throw UsageError("--raw and --json are mutually exclusive");
+    }
+
+    auto state = getEvalState();
+
+    auto v = installable->toValue(*state);
+    PathSet context;
+
+    if (raw) {
+      std::cout << state->coerceToString(noPos, *v, context);
+    } else if (json) {
+      JSONPlaceholder jsonOut(std::cout);
+      printValueAsJSON(*state, true, *v, jsonOut, context);
+    } else {
+      state->forceValueDeep(*v);
+      std::cout << *v << "\n";
+    }
+  }
+};
+}  // namespace nix
+
+static nix::RegisterCommand r1(nix::make_ref<nix::CmdEval>());