mondo: let expressions

packages/mondo/mondo.mjs

@@ -331,6 +331,19 @@ export class MondoRunner {
     //console.log(printAst(ast));
     return this.evaluate(ast, scope);
   }
+  evaluate_let(ast, scope) {
+    // (let ((x 3) (y 4)) ...body)
+    // = ((fn (x y) ...body) 3 4)
+    const defs = ast.children[1].children;
+    const args = defs.map((pair) => pair.children[0]);
+    const vals = defs.map((pair) => pair.children[1]);
+    const body = ast.children.slice(2);
+    const lambda = {
+      type: 'list',
+      children: [{ type: 'plain', value: 'fn' }, { type: 'list', children: args }, ...body],
+    };
+    return this.evaluate({ type: 'list', children: [lambda, ...vals] }, scope);
+  }
   evaluate_def(ast, scope) {
     // function definition special form?
     if (ast.children[1].type === 'list') {
@@ -441,6 +454,9 @@ export class MondoRunner {
     if (name === 'if') {
       return this.evaluate_if(ast, scope);
     }
+    if (name === 'let') {
+      return this.evaluate_let(ast, scope);
+    }
     if (name === 'def') {
       this.evaluate_def(ast, scope);
     }

packages/mondo/test/mondo.test.mjs

@@ -610,4 +610,47 @@ describe('mondo arithmetic', () => {
       ),
     ).toEqual(0.24998750000000042));
   it('sicp 84.1', () => expect(evaluate(`((fn (x y z) (+ x y (square z))) 1 2 3)`, scope)).toEqual(12));
+
+  // let expressions
+  it('sicp 87.1', () =>
+    expect(
+      evaluate(
+        `
+(+ (let ((x 3))
+(+ x (* x 10))) x)
+`,
+        { x: 5 },
+      ),
+    ).toEqual(38));
+  it('sicp 87.2', () =>
+    expect(
+      evaluate(
+        `
+(let ((x 3)
+(y (+ x 2)))
+(* x y))
+  `,
+        { x: 2 },
+      ),
+    ).toEqual(12));
+  it('sicp 88.1', () =>
+    expect(
+      evaluate(
+        `
+(def (f g) (g 2))
+(f square)
+      `,
+        scope,
+      ),
+    ).toEqual(4));
+  it('sicp 88.2', () =>
+    expect(
+      evaluate(
+        `
+(def (f g) (g 2))
+(f (fn (z) (* z (+ z 1))))
+          `,
+        scope,
+      ),
+    ).toEqual(6));
 });