cg/src/interpreter.ts

import type { AST, Pattern } from './ast';
import type { Env } from './env';
import type { Value } from './types';

export function evaluate(ast: AST, env: Env): Value {
    switch (ast.kind) {
        case 'literal':
            return ast.value;

        case 'variable': {
            const val = env.get(ast.name);

            if (val === undefined)
                throw new Error(`Unknown variable: ${ast.name}`);

            return val;
        }

        case 'list':
            return {
                kind: 'list',
                elements: ast.elements.map(el => evaluate(el, env))
            };

        case 'record':
            const fields: { [key: string]: Value } = {};

            Object.entries(ast.fields).forEach(([k, v]) => {
                fields[k] = evaluate(v, env);
            });

            return { kind: 'record', fields };

        case 'record-access': {
            const record = evaluate(ast.record, env);

            if (record.kind !== 'record')
                throw new Error('Not a record');

            const value = record.fields[ast.field];

            if (value === undefined) {
                throw new Error(`Field ${ast.field} not found`);
            }

            return value;
        }

        case 'record-update': {
            const record = evaluate(ast.record, env);

            if (record.kind !== 'record')
                throw new Error('Not a record');

            const newFields: { [key: string]: Value } = { ...record.fields };

            for (const [field, expr] of Object.entries(ast.updates)) {
                newFields[field] = evaluate(expr, env);
            }

            return { kind: 'record', fields: newFields };
        }

        case 'constructor':
            return {
                kind: 'constructor',
                name: ast.name,
                args: [] // TODO: constructors args
            };

        case 'let': {
            const newEnv = new Map(env);

            const val = evaluate(ast.value, newEnv);

            newEnv.set(ast.name, val);

            return evaluate(ast.body, newEnv);
        }

        case 'lambda':
            return {
                kind: 'closure',
                params: ast.params,
                body: ast.body,
                env
            }

        case 'apply': {
            // Operators
            if (ast.func.kind === 'variable') {
                const name = ast.func.name;
                const builtIns = ['+', '-', '*', '/', '>', '&'];

                if (builtIns.includes(name)) {
                    const argValues = ast.args.map(arg => evaluate(arg, env));

                    if (argValues.length !== 2) {
                        throw new Error(`${name} expects 2 args`);
                    }

                    return evaluateBuiltIn(name, argValues[0], argValues[1]);
                }
            }

            const func = evaluate(ast.func, env);

            if (func.kind !== 'closure')
                throw new Error('Not a function');

            const argValues = ast.args.map(arg => evaluate(arg, env));

            // Too few args (Currying)
            if (argValues.length < func.params.length) {
                // Bind the params we have
                const newEnv = new Map(func.env);

                for (let i = 0; i < argValues.length; i++) {
                    newEnv.set(func.params[i], argValues[i]);
                }

                return {
                    kind: 'closure',
                    params: func.params.slice(argValues.length),
                    body: func.body,
                    env: newEnv
                };
            }

            // Too many args
            if (argValues.length > func.params.length)
                throw new Error('Too many arguments');

            // Exact number of args
            const callEnv = new Map(func.env);
            for (let i = 0; i < argValues.length; i++) {
                callEnv.set(func.params[i], argValues[i]);
            }

            return evaluate(func.body, callEnv);
        }

        case 'match': {
            const value = evaluate(ast.expr, env);

            for (const matchCase of ast.cases) {
                const bindings = matchPattern(value, matchCase.pattern);

                if (bindings !== null) {
                    const newEnv = new Map(env);
                    for (const [name, val] of Object.entries(bindings)) {
                        newEnv.set(name, val);
                    }
                    return evaluate(matchCase.result, newEnv);
                }
            }

            throw new Error('Non-exhaustive pattern match');
        }

        default:
            throw new Error('Syntax Error');
    }
}

function evaluateBinaryOp(op: string, left: Value, right: Value): Value {
    const leftKind = left.kind;
    const rightKind = right.kind;

    const bothNumbers = ((leftKind === 'int' || leftKind === 'float')) && ((rightKind === 'int' || rightKind === 'float'));
    if (!bothNumbers)
        throw new Error(`Not numbers: ${left}, ${right}`);

    const leftValue = left.value;
    const rightValue = right.value;

    switch (op) {
        case '+':
            return { value: leftValue + rightValue, kind: 'int' };

        case '-':
            return { value: leftValue - rightValue, kind: 'int' };

        case '*':
            return { value: leftValue * rightValue, kind: 'int' }

        case '/':
            return { value: leftValue / rightValue, kind: 'int' }

        default:
                throw new Error(`Unknown operation: ${op}`);
    }
}

function evaluateBuiltIn(op: string, left: Value, right: Value): Value {
    if (op === '+' || op === '-' || op === '*' || op === '/') {
        return evaluateBinaryOp(op, left, right);
    }

    if (op === '>') {
        // x > f means f(x)
        if (right.kind !== 'closure')
            throw new Error('Right side of > must be a function');

        if (right.params.length !== 1)
            throw new Error('Pipe only works with 1-arg functions for now..');

        const callEnv = new Map(right.env);
        callEnv.set(right.params[0], left);
        return evaluate(right.body, callEnv);
    }

    if (op === '&') {
        if (left.kind == 'list' && right.kind === 'list') {
            return {
                kind: 'list',
                elements: [...left.elements, ...right.elements]
            };
        }

        if (left.kind == 'string' && right.kind === 'string') {
            return {
                kind: 'string',
                value: left.value + right.value
            };
        }
    }

    throw new Error(`Unknown built-in: ${op}`);
}

type Bindings = { [key: string]: Value };

function matchPattern(value: Value, pattern: Pattern): Bindings | null {
    switch (pattern.kind) {
        case 'wildcard':
            return {};

        case 'var':
            return { [pattern.name]: value };

        case 'literal':
            if (value.kind === 'int' || value.kind === 'float' || value.kind === 'string') {
                if (value.value === pattern.value) {
                    return {};
                }
            }
            return null;

        case 'constructor': {
            if (value.kind !== 'constructor') return null;
            if (value.name !== pattern.name) return null;
            if (value.args.length !== pattern.args.length) return null;

            const bindings: Bindings = {};
            for (let i = 0; i < pattern.args.length; i++) {
                const argBindings = matchPattern(value.args[i], pattern.args[i]);
                if (argBindings === null) return null;
                Object.assign(bindings, argBindings);
            }
            return bindings;
        }

        case 'list': {
            if (value.kind !== 'list') return null;
            if (value.elements.length !== pattern.elements.length) return null;

            const bindings: Bindings = {};
            for (let i = 0; i < pattern.elements.length; i++) {
                const elemBindings = matchPattern(value.elements[i], pattern.elements[i]);
                if (elemBindings === null) return null;
                Object.assign(bindings, elemBindings);
            }
            return bindings;
        }

        case 'record': {
            if (value.kind !== 'record') return null;

            const bindings: Bindings = {};
            for (const [fieldName, fieldPattern] of Object.entries(pattern.fields)) {
                const fieldValue = value.fields[fieldName];
                if (fieldValue === undefined) return null;

                const fieldBindings = matchPattern(fieldValue, fieldPattern);
                if (fieldBindings === null) return null;
                Object.assign(bindings, fieldBindings);
            }
            return bindings;
        }
    }

}