@maplibre/maplibre-gl-style-spec/src/expression/definitions/let.ts

a specification for maplibre styles

import type {Type} from '../types';
import type {Expression} from '../expression';
import type {ParsingContext} from '../parsing_context';
import type {EvaluationContext} from '../evaluation_context';

export class Let implements Expression {
    type: Type;
    bindings: Array<[string, Expression]>;
    result: Expression;

    constructor(bindings: Array<[string, Expression]>, result: Expression) {
        this.type = result.type;
        this.bindings = [].concat(bindings);
        this.result = result;
    }

    evaluate(ctx: EvaluationContext) {
        return this.result.evaluate(ctx);
    }

    eachChild(fn: (_: Expression) => void) {
        for (const binding of this.bindings) {
            fn(binding[1]);
        }
        fn(this.result);
    }

    static parse(args: ReadonlyArray<unknown>, context: ParsingContext): Expression {
        if (args.length < 4)
            return context.error(
                `Expected at least 3 arguments, but found ${args.length - 1} instead.`
            ) as null;

        const bindings: Array<[string, Expression]> = [];
        for (let i = 1; i < args.length - 1; i += 2) {
            const name = args[i];

            if (typeof name !== 'string') {
                return context.error(
                    `Expected string, but found ${typeof name} instead.`,
                    i
                ) as null;
            }

            if (/[^a-zA-Z0-9_]/.test(name)) {
                return context.error(
                    "Variable names must contain only alphanumeric characters or '_'.",
                    i
                ) as null;
            }

            const value = context.parse(args[i + 1], i + 1);
            if (!value) return null;

            bindings.push([name, value]);
        }

        const result = context.parse(
            args[args.length - 1],
            args.length - 1,
            context.expectedType,
            bindings
        );
        if (!result) return null;

        return new Let(bindings, result);
    }

    outputDefined() {
        return this.result.outputDefined();
    }
}