/* We're just going to take our string of code and break it down into an array * of tokens. * * (add 2 (subtract 4 2)) => [{ type: 'paren', value: '(' }, ...] */ // We start by accepting an input string of code, and we're gonna set up two // things... function tokenizer(input) { // A `current` variable for tracking our position in the code like a cursor. var current = 0; // And a `tokens` array for pushing our tokens to. var tokens = []; // We start by creating a `while` loop where we are setting up our `current` // variable to be incremented as much as we want `inside` the loop. // // We do this because we may want to increment `current` many times within a // single loop because our tokens can be any length. while (current < input.length) { // We're also going to store the `current` character in the `input`. var char = input[current]; // The first thing we want to check for is an open parenthesis. This will // later be used for `CallExpressions` but for now we only care about the // character. // // We check to see if we have an open parenthesis: if (char === '(') { // If we do, we push a new token with the type `paren` and set the value // to an open parenthesis. tokens.push({ type: 'paren', value: '(' }); // Then we increment `current` current++; // And we `continue` onto the next cycle of the loop. continue; } // Next we're going to check for a closing parenthesis. We do the same exact // thing as before: Check for a closing parenthesis, add a new token, // increment `current`, and `continue`. if (char === ')') { tokens.push({ type: 'paren', value: ')' }); current++; continue; } // Moving on, we're now going to check for whitespace. This is interesting // because we care that whitespace exists to separate characters, but it // isn't actually important for us to store as a token. We would only throw // it out later. // // So here we're just going to test for existence and if it does exist we're // going to just `continue` on. var WHITESPACE = /\s/; if (WHITESPACE.test(char)) { current++; continue; } // The next type of token is a number. This is different than what we have // seen before because a number could be any number of characters and we // want to capture the entire sequence of characters as one token. // // (add 123 456) // ^^^ ^^^ // Only two separate tokens // // So we start this off when we encounter the first number in a sequence. var NUMBERS = /[0-9]/; if (NUMBERS.test(char)) { // We're going to create a `value` string that we are going to push // characters to. var value = ''; // Then we're going to loop through each character in the sequence until // we encounter a character that is not a number, pushing each character // that is a number to our `value` and incrementing `current` as we go. while (NUMBERS.test(char)) { value += char; char = input[++current]; } // After that we push our `number` token to the `tokens` array. tokens.push({ type: 'number', value: value }); // And we continue on. continue; } // The last type of token will be a `name` token. This is a sequence of // letters instead of numbers, that are the names of functions in our lisp // syntax. // // (add 2 4) // ^^^ // Name token // var LETTERS = /[a-z]/i; if (LETTERS.test(char)) { var value = ''; // Again we're just going to loop through all the letters pushing them to // a value. while (LETTERS.test(char)) { value += char; char = input[++current]; } // And pushing that value as a token with the type `name` and continuing. tokens.push({ type: 'name', value: value }); continue; } // Finally if we have not matched a character by now, we're going to throw // an error and completely exit. throw new TypeError('I dont know what this character is: ' + char); } // Then at the end of our `tokenizer` we simply return the tokens array. return tokens; } /** * For our parser we're going to take our array of tokens and turn it into an * AST. * * [{ type: 'paren', value: '(' }, ...] => { type: 'Program', body: [...] } */ // Okay, so we define a `parser` function that accepts our array of `tokens`. function parser(tokens) { // Again we keep a `current` variable that we will use as a cursor. var current = 0; // But this time we're going to use recursion instead of a `while` loop. So we // define a `walk` function. function walk() { // Inside the walk function we start by grabbing the `current` token. var token = tokens[current]; // We're going to split each type of token off into a different code path, // starting off with `number` tokens. // // We test to see if we have a `number` token. if (token.type === 'number') { // If we have one, we'll increment `current`. current++; // And we'll return a new AST node called `NumberLiteral` and setting its // value to the value of our token. return { type: 'NumberLiteral', value: token.value }; } // Next we're going to look for CallExpressions. We start this off when we // encounter an open parenthesis. if ( token.type === 'paren' && token.value === '(' ) { // We'll increment `current` to skip the parenthesis since we don't care // about it in our AST. token = tokens[++current]; // We create a base node with the type `CallExpression`, and we're going // to set the name as the current token's value since the next token after // the open parenthesis is the name of the function. var node = { type: 'CallExpression', name: token.value, params: [] }; // We increment `current` *again* to skip the name token. token = tokens[++current]; // And now we want to loop through each token that will be the `params` of // our `CallExpression` until we encounter a closing parenthesis. // // Now this is where recursion comes in. Instead of trying to parse a // potentially infinitely nested set of nodes we're going to rely on // recursion to resolve things. // // To explain this, let's take our Lisp code. You can see that the // parameters of the `add` are a number and a nested `CallExpression` that // includes its own numbers. // // (add 2 (subtract 4 2)) // // You'll also notice that in our tokens array we have multiple closing // parenthesis. // // [ // { type: 'paren', value: '(' }, // { type: 'name', value: 'add' }, // { type: 'number', value: '2' }, // { type: 'paren', value: '(' }, // { type: 'name', value: 'subtract' }, // { type: 'number', value: '4' }, // { type: 'number', value: '2' }, // { type: 'paren', value: ')' }, <<< Closing parenthesis // { type: 'paren', value: ')' } <<< Closing parenthesis // ] // // We're going to rely on the nested `walk` function to increment our // `current` variable past any nested `CallExpressions`. // So we create a `while` loop that will continue until it encounters a // token with a `type` of `'paren'` and a `value` of a closing // parenthesis. while ( (token.type !== 'paren') || (token.type === 'paren' && token.value !== ')') ) { // we'll call the `walk` function which will return a `node` and we'll // push it into our `node.params`. node.params.push(walk()); token = tokens[current]; } // Finally we will increment `current` one last time to skip the closing // parenthesis. current++; // And return the node. return node; } // Again, if we haven't recognized the token type by now we're going to // throw an error. throw new TypeError(token.type); } // Now, we're going to create our AST which will have a root which is a // `Program` node. var ast = { type: 'Program', body: [] }; // And we're going to kickstart our `walk` function, pushing nodes to our // `ast.body` array. // // The reason we are doing this inside a loop is because our program can have // `CallExpressions` after one another instead of being nested. // // (add 2 2) // (subtract 4 2) // while (current < tokens.length) { ast.body.push(walk()); } // At the end of our parser we'll return the AST. return ast; } // So we define a traverser function which accepts an AST and a // visitor. Inside we're going to define two functions... function traverser(ast, visitor) { // A `traverseArray` function that will allow us to iterate over an array and // call the next function that we will define: `traverseNode`. function traverseArray(array, parent) { array.forEach(function(child) { traverseNode(child, parent); }); } // `traverseNode` will accept a `node` and its `parent` node. So that it can // pass both to our visitor methods. function traverseNode(node, parent) { // We start by testing for the existence of a method on the visitor with a // matching `type`. var method = visitor[node.type]; // If it exists we'll call it with the `node` and its `parent`. if (method) { method(node, parent); } // Next we are going to split things up by the current node type. switch (node.type) { // We'll start with our top level `Program`. Since Program nodes have a // property named body that has an array of nodes, we will call // `traverseArray` to traverse down into them. // // (Remember that `traverseArray` will in turn call `traverseNode` so we // are causing the tree to be traversed recursively) case 'Program': traverseArray(node.body, node); break; // Next we do the same with `CallExpressions` and traverse their `params`. case 'CallExpression': traverseArray(node.params, node); break; // In the case of `NumberLiterals` we don't have any child nodes to visit, // so we'll just break. case 'NumberLiteral': break; // And again, if we haven't recognized the node type then we'll throw an // error. default: throw new TypeError(node.type); } } // Finally we kickstart the traverser by calling `traverseNode` with our ast // with no `parent` because the top level of the AST doesn't have a parent. traverseNode(ast, null); } // So we have our transformer function which will accept the lisp ast. function transformer(ast) { // We'll create a `newAst` which like our previous AST will have a program // node. var newAst = { type: 'Program', body: [] }; // Next I'm going to cheat a little and create a bit of a hack. We're going to // use a property named `context` on our parent nodes that we're going to push // nodes to their parent's `context`. Normally you would have a better // abstraction than this, but for our purposes this keeps things simple. // // Just take note that the context is a reference *from* the old ast *to* the // new ast. ast._context = newAst.body; // We'll start by calling the traverser function with our ast and a visitor. traverser(ast, { // The first visitor method accepts `NumberLiterals` NumberLiteral: function(node, parent) { // We'll create a new node also named `NumberLiteral` that we will push to // the parent context. parent._context.push({ type: 'NumberLiteral', value: node.value }); }, // Next up, `CallExpressions`. CallExpression: function(node, parent) { // We start creating a new node `CallExpression` with a nested // `Identifier`. var expression = { type: 'CallExpression', callee: { type: 'Identifier', name: node.name }, arguments: [] }; // Next we're going to define a new context on the original // `CallExpression` node that will reference the `expression`'s arguments // so that we can push arguments. node._context = expression.arguments; // Then we're going to check if the parent node is a `CallExpression`. // If it is not... if (parent.type !== 'CallExpression') { // We're going to wrap our `CallExpression` node with an // `ExpressionStatement`. We do this because the top level // `CallExpressions` in JavaScript are actually statements. expression = { type: 'ExpressionStatement', expression: expression }; } // Last, we push our (possibly wrapped) `CallExpression` to the `parent`'s // `context`. parent._context.push(expression); } }); // At the end of our transformer function we'll return the new ast that we // just created. return newAst; } function printAST(ast) { function traverseArray(array, parent, depth) { array.forEach(function(child) { traverseNode(child, parent,depth); }); } function traverseNode(node, parent, depth) { var spaces = ""; for(let i=0; i < depth; i ++) { spaces = spaces + "\t"; } // Next we are going to split things up by the current node type. switch (node.type) { case 'Program': console.log(spaces, node.type); traverseArray(node.body, node, depth+1); break; case 'CallExpression': console.log(spaces, node.type, ' - ', node.name); traverseArray(node.params, node, depth+1); break; case 'NumberLiteral': console.log(spaces, node.type,' - ', node.value); break; default: throw new TypeError(node.type); } } traverseNode(ast, null, 0); } function printNewAST(ast) { function traverseArray(array, parent, depth) { array.forEach(function(child) { traverseNode(child, parent,depth); }); } function traverseNode(node, parent, depth) { let spaces = ""; for(let i=0; i < depth; i ++) { spaces = spaces + "\t"; } // Next we are going to split things up by the current node type. switch (node.type) { case 'Program': console.log(spaces, node.type); traverseArray(node.body, node, depth+1); break; case 'CallExpression': console.log(spaces, node.type); traverseArray([node.callee], node, depth+1); traverseArray(node.arguments, node, depth+1); break; case 'ExpressionStatement': console.log(spaces, node.type); traverseArray([node.expression], node, depth+1); break; case 'Identifier': console.log(spaces, node.type, ' - ', node.name); break; case 'NumberLiteral': console.log(spaces, node.type,' - ', node.value); break; default: throw new TypeError(node.type); } } traverseNode(ast, null, 0); } function codeGenerator(node) { // We'll break things down by the `type` of the `node`. switch (node.type) { // If we have a `Program` node. We will map through each node in the `body` // and run them through the code generator and join them with a newline. case 'Program': return node.body.map(codeGenerator) .join('\n'); // For `ExpressionStatements` we'll call the code generator on the nested // expression and we'll add a semicolon... case 'ExpressionStatement': return ( codeGenerator(node.expression) + ';' // << (...because we like to code the *correct* way) ); // For `CallExpressions` we will print the `callee`, add an open // parenthesis, we'll map through each node in the `arguments` array and run // them through the code generator, joining them with a comma, and then // we'll add a closing parenthesis. case 'CallExpression': return ( codeGenerator(node.callee) + '(' + node.arguments.map(codeGenerator) .join(', ') + ')' ); // For `Identifiers` we'll just return the `node`'s name. case 'Identifier': return node.name; // For `NumberLiterals` we'll just return the `node`'s value. case 'NumberLiteral': return node.value; // And if we haven't recognized the node, we'll throw an error. default: throw new TypeError(node.type); } }

Complete compiler