Ch9 & Ch11

Author: ankt271192

Chapter11/Arrays.js

@@ -0,0 +1,175 @@
+function parseExpression(program) {
+  program = skipSpace(program);
+  var match, expr;
+  if (match = /^"([^"]*)"/.exec(program))
+    expr = {type: "value", value: match[1]};
+  else if (match = /^\d+\b/.exec(program))
+    expr = {type: "value", value: Number(match[0])};
+  else if (match = /^[^\s(),"]+/.exec(program))
+    expr = {type: "word", name: match[0]};
+  else
+    throw new SyntaxError("Unexpected syntax: " + program);
+
+  return parseApply(expr, program.slice(match[0].length));
+}
+
+function skipSpace(string) {
+  var first = string.search(/\S/);
+  if (first == -1) return "";
+  return string.slice(first);
+}
+
+function parseApply(expr, program) {
+  program = skipSpace(program);
+  if (program[0] != "(")
+    return {expr: expr, rest: program};
+
+  program = skipSpace(program.slice(1));
+  expr = {type: "apply", operator: expr, args: []};
+  while (program[0] != ")") {
+    var arg = parseExpression(program);
+    expr.args.push(arg.expr);
+    program = skipSpace(arg.rest);
+    if (program[0] == ",")
+      program = skipSpace(program.slice(1));
+    else if (program[0] != ")")
+      throw new SyntaxError("Expected ',' or ')'");
+  }
+  return parseApply(expr, program.slice(1));
+}
+
+function parse(program) {
+  var result = parseExpression(program);
+  if (skipSpace(result.rest).length > 0)
+    throw new SyntaxError("Unexpected text after program");
+  return result.expr;
+}
+
+function evaluate(expr, env) {
+  switch(expr.type) {
+    case "value":
+      return expr.value;
+
+    case "word":
+      if (expr.name in env)
+        return env[expr.name];
+      else
+        throw new ReferenceError("Undefined variable: " +
+                                 expr.name);
+    case "apply":
+      if (expr.operator.type == "word" &&
+          expr.operator.name in specialForms)
+        return specialForms[expr.operator.name](expr.args,
+                                                env);
+      var op = evaluate(expr.operator, env);
+      if (typeof op != "function")
+        throw new TypeError("Applying a non-function.");
+      return op.apply(null, expr.args.map(function(arg) {
+        return evaluate(arg, env);
+      }));
+  }
+}
+
+var specialForms = Object.create(null);
+
+specialForms["if"] = function(args, env) {
+  if (args.length != 3)
+    throw new SyntaxError("Bad number of args to if");
+
+  if (evaluate(args[0], env) !== false)
+    return evaluate(args[1], env);
+  else
+    return evaluate(args[2], env);
+};
+
+specialForms["while"] = function(args, env) {
+  if (args.length != 2)
+    throw new SyntaxError("Bad number of args to while");
+
+  while (evaluate(args[0], env) !== false)
+    evaluate(args[1], env);
+
+  // Since undefined does not exist in Egg, we return false,
+  // for lack of a meaningful result.
+  return false;
+};
+
+specialForms["do"] = function(args, env) {
+  var value = false;
+  args.forEach(function(arg) {
+    value = evaluate(arg, env);
+  });
+  return value;
+};
+
+specialForms["define"] = function(args, env) {
+  if (args.length != 2 || args[0].type != "word")
+    throw new SyntaxError("Bad use of define");
+  var value = evaluate(args[1], env);
+  env[args[0].name] = value;
+  return value;
+};
+
+var topEnv = Object.create(null);
+
+topEnv["true"] = true;
+topEnv["false"] = false;
+
+["+", "-", "*", "/", "==", "<", ">"].forEach(function(op) {
+  topEnv[op] = new Function("a, b", "return a " + op + " b;");
+});
+
+topEnv["print"] = function(value) {
+  console.log(value);
+  return value;
+};
+
+function run() {
+  var env = Object.create(topEnv);
+  var program = Array.prototype.slice
+    .call(arguments, 0).join("\n");
+  return evaluate(parse(program), env);
+}
+
+specialForms["fun"] = function(args, env) {
+  if (!args.length)
+    throw new SyntaxError("Functions need a body");
+  function name(expr) {
+    if (expr.type != "word")
+      throw new SyntaxError("Arg names must be words");
+    return expr.name;
+  }
+  var argNames = args.slice(0, args.length - 1).map(name);
+  var body = args[args.length - 1];
+
+  return function() {
+    if (arguments.length != argNames.length)
+      throw new TypeError("Wrong number of arguments");
+    var localEnv = Object.create(env);
+    for (var i = 0; i < arguments.length; i++)
+      localEnv[argNames[i]] = arguments[i];
+    return evaluate(body, localEnv);
+  };
+};
+
+topEnv["array"] = function() {
+  return Array.prototype.slice.call(arguments, 0);
+};
+
+topEnv["length"] = function(array) {
+  return array.length;
+};
+
+topEnv["element"] = function(array, n) {
+  return array[n];
+};
+
+run("do(define(sum, fun(array,",
+    "     do(define(i, 0),",
+    "        define(sum, 0),",
+    "        while(<(i, length(array)),",
+    "          do(define(sum, +(sum, element(array, i))),",
+    "             define(i, +(i, 1)))),",
+    "        sum))),",
+    "   print(sum(array(1, 2, 3))))");
+// → 6

Chapter11/Comments.js

@@ -0,0 +1,54 @@
+function parseExpression(program) {
+  program = skipSpace(program);
+  var match, expr;
+  if (match = /^"([^"]*)"/.exec(program))
+    expr = {type: "value", value: match[1]};
+  else if (match = /^\d+\b/.exec(program))
+    expr = {type: "value", value: Number(match[0])};
+  else if (match = /^[^\s(),"]+/.exec(program))
+    expr = {type: "word", name: match[0]};
+  else
+    throw new SyntaxError("Unexpected syntax: " + program);
+
+  return parseApply(expr, program.slice(match[0].length));
+}
+
+function parseApply(expr, program) {
+  program = skipSpace(program);
+  if (program[0] != "(")
+    return {expr: expr, rest: program};
+
+  program = skipSpace(program.slice(1));
+  expr = {type: "apply", operator: expr, args: []};
+  while (program[0] != ")") {
+    var arg = parseExpression(program);
+    expr.args.push(arg.expr);
+    program = skipSpace(arg.rest);
+    if (program[0] == ",")
+      program = skipSpace(program.slice(1));
+    else if (program[0] != ")")
+      throw new SyntaxError("Expected ',' or ')'");
+  }
+  return parseApply(expr, program.slice(1));
+}
+
+function parse(program) {
+  var result = parseExpression(program);
+  if (skipSpace(result.rest).length > 0)
+    throw new SyntaxError("Unexpected text after program");
+  return result.expr;
+}
+
+
+function skipSpace(string) {
+  var skip = /^(\s|#.*)*/.exec(string);
+  return string.slice(skip[0].length);
+}
+
+console.log(parse("# hello\nx"));
+// → {type: "word", name: "x"}
+
+console.log(parse("a # one\n   # two\n()"));
+// → {type: "apply",
+//    operator: {type: "word", name: "a"},
+//    args: []}