1 files changed, 106 insertions, 0 deletions
diff --git a/repl.ts b/repl.ts
new file mode 100644
index 0000000..6a67e50
--- /dev/null
+++ b/repl.ts
@@ -0,0 +1,106 @@
+const peggy = require("./parser.js");
+
+const isVariable = (ast: any) => typeof ast === "string";
+const isApplication = (ast: any) =>
+  typeof ast === "object" && "application" in ast;
+const isAbstraction = (ast: any) =>
+  typeof ast === "object" && "abstraction" in ast;
+
+const emitCode = (ast: any) => {
+  if (isVariable(ast)) {
+    return ast;
+  }
+
+  if (isApplication(ast)) {
+    const { application } = ast;
+    const { left, right } = application;
+    return `( ${emitCode(left)} ${emitCode(right)} )`;
+  }
+
+  const { abstraction } = ast;
+  const { param, body } = abstraction;
+
+  return `(λ ${emitCode(param)} . ${emitCode(body)})`;
+};
+
+const substitute = (param: string, term: any, result: any) => {
+  if (isVariable(term)) {
+    if (term === param) {
+      return result;
+    }
+    return term;
+  }
+
+  if (isApplication(term)) {
+    const { application } = term;
+    const { left, right } = application;
+
+    return {
+      application: {
+        left: substitute(param, left, result),
+        right: substitute(param, right, result),
+      },
+    };
+  }
+
+  const { abstraction } = term;
+  const { body } = abstraction;
+  return {
+    abstraction: {
+      param: abstraction.param,
+      body: substitute(param, body, result),
+    },
+  };
+};
+
+const betaReduce = (ast: any) => {
+  if (isVariable(ast)) {
+    return ast;
+  }
+
+  if (isApplication(ast)) {
+    const { application } = ast;
+    const { left, right } = application;
+
+    if (isAbstraction(left)) {
+      const { abstraction } = left;
+      const { param } = abstraction;
+
+      return substitute(param, right);
+    }
+
+    return {
+      application: {
+        left: betaReduce(left),
+        right: betaReduce(right),
+      },
+    };
+  }
+
+  // it's an abstraction
+  const { abstraction } = ast;
+  const { param, body } = abstraction;
+  return { param: betaReduce(param), body: betaReduce(body) };
+};
+
+const evaluate = (lambdaTerm: string) => {
+  const ast = peggy.parse(lambdaTerm);
+
+  const fullBetaReduction = betaReduce(ast);
+  return emitCode(fullBetaReduction);
+};
+
+const doRepl = async () => {
+  const prompt = ">>> ";
+  process.stdout.write(prompt);
+
+  for await (const line of console) {
+    const result = evaluate(line);
+    console.log(result);
+    break;
+  }
+
+  await doRepl();
+};
+
+await doRepl();