JavaScript Playground

function tokenizeScheme(input) {
    const tokens = [];
    let current = 0;

const isWhitespace = (char) => /\s/.test(char);
    const isDigit = (char) => /[0-9]/.test(char);
    const isParen = (char) => char === '(' || char === ')';
    // Symbols can include letters, numbers, and some punctuation like - _ ! ?
    const isSymbolChar = (char) => /[a-zA-Z0-9\+\-\*\/\=\?\!\_]/.test(char);

while (current < input.length) {
        let char = input[current];

if (isWhitespace(char)) {
            current++;
            continue;
        }

if (isParen(char)) {
            tokens.push({ type: 'paren', value: char });
            current++;
            continue;
        }

if (isDigit(char) || (char === '-' && isDigit(input[current + 1]))) {
            let number = '';
            while (isDigit(char) || char === '-') {
                number += char;
                char = input[++current];
            }
            tokens.push({ type: 'number', value: number });
            continue;
        }

// Handle symbols, including letters, numbers, punctuation
        if (isSymbolChar(char)) {
            let symbol = '';
            while (isSymbolChar(char)) {
                symbol += char;
                char = input[++current];
            }
            tokens.push({ type: 'symbol', value: symbol });
            continue;
        }

throw new Error(`Unexpected character: ${char}`);
    }

return tokens;
}

function parseScheme(tokens) {
    let current = 0;

function walk() {
        let token = tokens[current];

if (token.type === 'number') {
            current++;
            return { type: 'NumberLiteral', value: Number(token.value) };
        }

if (token.type === 'symbol') {
            current++;
            return { type: 'Symbol', value: token.value };
        }

if (token.type === 'paren' && token.value === '(') {
            current++;
            const node = { type: 'List', value: [] };

while (!(tokens[current].type === 'paren' && tokens[current].value === ')')) {
                node.value.push(walk());
            }

current++; // Skip closing ')'
            return node;
        }

throw new Error(`Unexpected token: ${token.type}`);
    }

return walk();
}

const globalEnv = {
    '+': (...args) => args.reduce((acc, curr) => acc + curr),
    '-': (...args) => args.reduce((acc, curr) => acc - curr),
    '*': (...args) => args.reduce((acc, curr) => acc * curr),
    '/': (a, b) => a / b, // Only two arguments for division...right?
    'eq?': (...args) => args.every((val, i, arr) => val === arr[0]),
    'car': (list) => list[0],
    'cdr': (list) => list.slice(1),
    'cons': (a, b) => [a].concat(b),
    'null?': (list) => list.length === 0,
};

function evaluate(node, env = globalEnv) {
    if (node.type === 'NumberLiteral') {
        return node.value;
    }

if (node.type === 'Symbol') {
        if (env[node.value] !== undefined) {
            return env[node.value];
        }
        throw new Error(`Undefined symbol: ${node.value}`);
    }

if (node.type === 'List') {
        const [first, ...rest] = node.value;

// Is the first element a symbol, like an operator or function name?
        if (first.type === 'Symbol') {
            const operator = first.value;

// Special case for define
            if (operator === 'define') {
                const [symbol, expr] = rest;
                env[symbol.value] = evaluate(expr, env);
                return;
            }

// Special case for lambda
            if (operator === 'lambda') {
                const [params, body] = rest;

// Create a closure to return
                return function (...args) {
                    const lambdaEnv = { ...env };

// Bind each argument to the corresponding parameter...
                    params.value.forEach((param, i) => {
                        lambdaEnv[param.value] = args[i];
                    });

// ...and then evaluate the body with the environment
                    return evaluate(body, lambdaEnv);
                };
            }

// Special case for if
            if (operator === 'if') {
                const [test, consequent, alternate] = rest;
                const condition = evaluate(test, env);
                return condition ? evaluate(consequent, env) : evaluate(alternate, env);
            }
        }

// Evaluate the first element
        const func = evaluate(first, env);

// Check that `func` is a function. If it isn't, don't try to apply it
        if (typeof func !== 'function') {
            throw new Error(`Expected a function but got: ${func}`);
        }

const args = rest.map(arg => evaluate(arg, env));
        return func(...args);
    }

throw new Error(`Unexpected node type: ${node.type}`);
}

function evalScheme(input) {
    const tokens = tokenizeScheme(input);
    const ast = parseScheme(tokens);
    return evaluate(ast);
}

/* 
console.log(evalScheme("(define add1 (lambda (x) (+ x 1)))"));
console.log(evalScheme("(add1 10)"));
console.log(evalScheme("(eq? 1 1)"));
console.log(evalScheme("(eq? 1 2)"));
console.log(evalScheme("(+ 1 2 3 6)"));
*/

function mountRepl(playground) {
    // Create a REPL container
    const replContainer = document.createElement('div');
    replContainer.style.display = 'flex';
    replContainer.style.flexDirection = 'column';
    replContainer.style.width = '100%';

// Create an input field for the Scheme expressions
    const input = document.createElement('textarea');
    input.placeholder = "Scheme here...";
    input.style.width = '100%';
    input.style.height = '100px';
    input.style.marginBottom = '10px';
    input.style.fontFamily = 'monospace';

// Create a button to evaluate the expression
    const evalButton = document.createElement('button');
    evalButton.textContent = 'Evaluate';

// Create a container to display the results
    const output = document.createElement('pre');
    output.style.width = '100%';
    output.style.height = '200px';
    output.style.overflowY = 'auto';
    output.style.backgroundColor = '#f0f0f0';
    output.style.padding = '10px';
    output.style.fontFamily = 'monospace';

// Add the input, button, and output to the REPL container
    replContainer.appendChild(input);
    replContainer.appendChild(evalButton);
    replContainer.appendChild(output);

// Add the REPL container to the playground div
    playground.appendChild(replContainer);

evalButton.addEventListener('click', () => {
        const expression = input.value.trim();
        if (expression) {
            try {
                // Evaluate the expression
                const result = evalScheme(expression);
                // Append the result to the output area
                output.textContent += `> ${expression}\n${result}\n\n`;
            } catch (error) {
                // Error if the expression is invalid
                output.textContent += `> ${expression}\nError: ${error.message}\n\n`;
            }
        }
        // Clear input after evaluation
        input.value = '';
    });
}

mount(mountRepl);