Function subsetSystem

article/utils.ts:476–497

(
  node: Term,
  keepTypes: Types[],
  keepTerms: Terms[],
)

type ExtractTags<T, K> = Extract<{ [P in keyof T]: ExtractTagsSub<T[P], K> }, { tag: K }>;

function subsetSystem<Types extends Type["tag"], Terms extends Term["tag"]>(
  node: Term,
  keepTypes: Types[],
  keepTerms: Terms[],
): ExtractTags<Term, Types | Terms> {
  if (!node.tag) return node;

  if (!(<string[]> keepTypes).includes(node.tag) && !(<string[]> keepTerms).includes(node.tag)) {
    throw new Error(`"${node.tag}" is not allowed in this system`);
  }

  // deno-lint-ignore no-explicit-any
  const newNode: any = {};
  Object.entries(node).forEach(([key, val]) => {
    if (typeof val !== "object" || !val.tag) {
      newNode[key] = val;
    } else {
      newNode[key] = subsetSystem<Types, Terms>(val, keepTypes, keepTerms);
    }
  });
  return newNode;
}

export function parse(code: string): Term {
  const node = p.parse(code, { allowInvalidAST: false, loc: true });