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| 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(left, program) { | |
| program = skipSpace(program); | |
| if (program[0] != "(") | |
| return {expr: left, rest: program}; | |
| program = skipSpace(program.slice(1)); | |
| var expr = {type: "apply", operator: left, 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 of 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 exit in out language, and this form does not | |
| // return a meaningful value, we return false. | |
| 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[arg[0].name] = value; | |
| return value; | |
| }; | |
| 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); | |
| }; | |
| }; | |
| specialForms["set"] = function(args, env) { | |
| if (args.length != 2 || args[0].type != "word") | |
| throw new SyntaxError("bad use of set"); | |
| var value = evaluate(args[1], env); | |
| while (env) { | |
| if (Object.prototype.hasOwnProperty.call(env, args[0].name)) { | |
| env[args[0].name] = value; | |
| return value; | |
| } | |
| env = Object.getPrototypeOf(env); | |
| } | |
| throw new ReferenceError("Undefined variable: " + args[0].name); | |
| } | |
| 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; | |
| }; | |
| topEnv["array"] = function(args) { | |
| var array = []; | |
| for (var i = arguments.length - 1; i >= 0; i--) { | |
| array.push(arguments[i]); | |
| }; | |
| return array; | |
| }; | |
| topEnv["length"] = function(array) { | |
| return array.length; | |
| }; | |
| topEnv["element"] = function(array, index) { | |
| return array[index]; | |
| } | |
| function run() { | |
| var env = Object.create(topEnv); | |
| var program = Array.prototype.slice.call(arguments, 0).join("\n"); | |
| return evaluate(parse(program), env); | |
| } |
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