1 files changed, 180 insertions, 0 deletions
diff --git a/articles/programming-languages-code.js b/articles/programming-languages-code.js
new file mode 100644
index 0000000..cb7d764
--- /dev/null
+++ b/articles/programming-languages-code.js
@@ -0,0 +1,180 @@
+/* function evaluate(node) {
+  switch (node.type) {
+    case "HelloWorld":
+      return "Hello, world!";
+    default:
+      throw `evaluate -- unknown node type ${node.type}`;
+  }
+}
+
+const program = { type: "HelloWorld" };
+console.log(evaluate(program));
+// => Hello, world!
+
+console.log(evaluate({ type: "Blah" }));
+
+function evaluate(node) {
+  switch (node.type) {
+    case "String":
+      return node.content;
+    case "Excite":
+      return evaluate(node.expression) + "!";
+    default:
+      throw `evaluate -- unknown node type ${node.type}`;
+  }
+}
+
+console.log(
+  evaluate({
+    type: "String",
+    content: "Apple"
+  })
+);
+// => Apple
+
+console.log(
+  evaluate({
+    type: "Excite",
+    expression: {
+      type: "Excite",
+      expression: {
+        type: "String",
+        content: "Banana"
+      }
+    }
+  })
+);
+// => Banana!!
+
+console.log(
+  evaluate({
+    type: "Excite",
+    expression: {
+      type: "String",
+      content: "Banana"
+    }
+  })
+);
+// => Banana!
+
+console.log(
+  evaluate({
+    type: "Append",
+    first: {
+      type: "String",
+      content: "Apple"
+    },
+    second: {
+      type: "Excite",
+      expression: {
+        type: "String",
+        content: "Banana"
+      }
+    }
+  })
+);
+// => AppleBanana! */
+
+function lookup(env, name) {
+  return env[name];
+}
+
+function extendEnv(env, name, value) {
+  return {
+    ...env,
+    [name]: value,
+  };
+}
+
+function evaluate(node, env) {
+  switch (node.type) {
+    case "String":
+      return node.content;
+    case "Excite":
+      return evaluate(node.expression, env) + "!";
+    case "Append":
+      return evaluate(node.first, env) + evaluate(node.second, env);
+    case "Variable":
+      return lookup(env, node.name);
+    case "Let":
+      let inner = node.expression;
+      let value = evaluate(node.value, env);
+      let newEnv = extendEnv(env, node.name, value);
+
+      return evaluate(node.expression, newEnv);
+    default:
+      throw `evaluate -- unknown node type ${node.type}`;
+  }
+}
+
+console.log(
+  evaluate({
+    type: "String",
+    content: "Apple",
+  })
+);
+// => Apple
+
+console.log(
+  evaluate({
+    type: "Excite",
+    expression: {
+      type: "String",
+      content: "Banana",
+    },
+  })
+);
+// => Banana!
+
+console.log(
+  evaluate({
+    type: "Append",
+    first: {
+      type: "String",
+      content: "Apple",
+    },
+    second: {
+      type: "Excite",
+      expression: {
+        type: "String",
+        content: "Banana",
+      },
+    },
+  })
+);
+// => AppleBanana!
+
+console.log(
+  evaluate(
+    {
+      type: "Let",
+      name: "x",
+      value: {
+        type: "String",
+        content: "Hello, world",
+      },
+      expression: {
+        type: "Excite",
+        expression: {
+          type: "Variable",
+          name: "x",
+        },
+      },
+    },
+    {}
+  )
+);
+// => Hello, world!
+
+console.log(
+  evaluate({
+    type: "Excite",
+    expression: {
+      type: "Excite",
+      expression: {
+        type: "String",
+        content: "Banana",
+      },
+    },
+  })
+);