add REPL.

(it's not done yet, but the entry point already imports this since like
2 commits back.)

src/repl.mjs

@@ -0,0 +1,45 @@
+import Line from "./Line.mjs";
+import { parse_line } from "./parse.mjs";
+import { compile_line } from "./compile.mjs";
+import { execute } from "./execute.mjs";
+
+
+
+let env = new Map();
+
+function run (line) {
+	const parsed = parse_line(line);
+	const type = parsed.type;
+
+	const compiled = compile_line(env, parsed);
+
+	if (!compiled.valid) {
+		console.log("invalid line!");
+		return;
+	}
+
+	env = compiled.env;
+
+	if (type === Line.Expression) {
+		console.log(prettify(evaluate(compiled.value)));
+	} else {
+		console.log(env);
+	}
+}
+
+function evaluate (fn) {
+	return execute(fn[0])(fn[1]);
+}
+
+function prettify (code) {
+	if (Array.isArray(code)) {
+		return prettify(code[0]).concat(" ", prettify(code[1]));
+	}
+	if (code === 0) {
+		return "λ";
+	}
+	return code.toString();
+}
+
+run("id = a → a");
+run("id id");