dope2/structures/enum.js

import { Bottom } from "../primitives/types.js";
import { capitalizeFirstLetter } from "../util/util.js";
import { newProduct as newProduct, getLeft, getRight } from "./product.js";
import { newLeft, newRight, match } from "./sum.js";
import { sumType } from "./types.js";

// 'variants' is an array of (name: string, type: Type) pairs.
// e.g., the list of variants:
//   [ { l: "price"    , r: Int   },
//     { l: "prices"   , r: [Int] },
//     { l: "not_found", r: Unit  }  ]
// results in the type:
//   (Int | ([Int] | (Unit | ⊥)))
export const enumType = variants => {
  if (variants.length === 0) {
    return Bottom; // empty enum is equal to Bottom-type (cannot be instantiated)
  }
  const [variant, ...rest] = variants;
  const variantType = getRight(variant);
  return sumType(() => variantType)(() => enumType(rest));
};

const eatParameters = (numParams, result) => {
  if (numParams === 0) {
    return result;
  }
  else return () => eatParameters(numParams-1, result);
}

export const makeMatchFn = variants => {
  if (variants.length === 0) {
    return undefined;
  }
  const [_, ...remainingVariants] = variants;
  return sum => handler => {
    return (
      match(sum)(newProduct
        (leftValue => eatParameters(remainingVariants.length, handler(leftValue)))
        (rightValue => makeMatchFn(remainingVariants)(rightValue))
      ));
  };
};

export const makeConstructors = variants => {
  if (variants.length === 0) {
    return [];
  }
  const [variant, ...remainingVariants] = variants;
  const name = getLeft(variant);
  const ctorName = `new${capitalizeFirstLetter(name)}`;
  const constructor = { [ctorName]: val => newLeft(val) }[ctorName];
  return [
    constructor,
    ...makeConstructors(remainingVariants).map(ctor =>
      ({[ctor.name]: val => newRight(ctor(val))}[ctor.name])),
  ];
}