moss/src/ast/mixin.ts

import * as AST from '../ast';
import Env, { EnvError } from '../env';
import Token from '../token';

interface Expansion {
  rules: (String | EnvError)[];
  children: (String | EnvError)[];
}

export class Mixin {
  public name: Token;
  public parameters: AST.Identifier[];
  public children: AST.Child[];

  public constructor(
    name: Token,
    parameters: AST.Identifier[],
    children: AST.Child[]
  ) {
    this.name = name;
    this.parameters = parameters;
    this.children = children;
  }

  public compile(env: Env, _opts: AST.Opts) {
    env.set(this.name.value, this);
    return '';
  }

  public expand(
    env: Env,
    opts: AST.Opts,
    parents: AST.Selector[],
    args: AST.Node[]
  ): Expansion | EnvError {
    if (this.parameters.length !== args.length) {
      return new EnvError(
        this.name.line,
        `Expected ${this.parameters.length} arguments but received ${
          args.length
        }`
      );
    }

    const mixinEnv = new Env(env);
    this.parameters.forEach((param, index) => {
      mixinEnv.set(param.name.value, args[index]);
    });

    const rules = this.children
      .filter((child: AST.Node): child is AST.Rule => child instanceof AST.Rule)
      .map((child: AST.Rule) => child.compile(mixinEnv, opts));

    const children = this.children
      .filter(
        (child: AST.Node): child is AST.RuleSet => child instanceof AST.RuleSet
      )
      .map((child: AST.RuleSet) => {
        child.selectors.forEach((sel) => (sel.parents = parents));
        return child.compile(mixinEnv, opts);
      });

    return { rules, children };
  }
}