Initial version of nix-repl

This program interactively reads a Nix expression from the user,
evaluates it, and prints the result.

default.nix

@@ -0,0 +1,10 @@
+with import <nixpkgs> { };
+
+runCommand "nix-repl"
+  { buildInputs = [ readline nixUnstable boehmgc ]; }
+  ''
+    mkdir -p $out/bin
+    g++ -O3 -Wall -o $out/bin/nix-repl ${./nix-repl.cc} \
+      -I${nixUnstable}/include/nix -L${nixUnstable}/lib/nix \
+      -lexpr -lmain -lreadline
+  ''

nix-repl.cc

@@ -0,0 +1,57 @@
+#include <iostream>
+#include <cstdlib>
+
+#include <readline/readline.h>
+#include <readline/history.h>
+
+#include "shared.hh"
+#include "eval.hh"
+
+using namespace std;
+using namespace nix;
+
+
+string programId = "nix-repl";
+
+
+void printHelp()
+{
+    std::cout << "Usage: nix-repl\n";
+}
+
+
+bool getLine(string & line)
+{
+    char * s = readline ("nix-repl> ");
+    if (!s) return false;
+    line = chomp(string(s));
+    free(s);
+    if (line != "") add_history(line.c_str());
+    return true;
+}
+
+
+void run(nix::Strings args)
+{
+    EvalState state;
+    Path curDir = absPath(".");
+
+    while (true) {
+        string line;
+        if (!getLine(line)) break;
+
+        try {
+            Expr * e = state.parseExprFromString(line, curDir);
+            Value v;
+            state.eval(e, v);
+            state.strictForceValue(v);
+            std::cout << v << std::endl;
+        } catch (Error & e) {
+            printMsg(lvlError, e.msg());
+        }
+
+        std::cout << std::endl;
+    }
+
+    std::cout << std::endl;
+}