Typescript match discriminated union tag

matchTag.ts

type MatchTag = {
  // Strict match, one handler must be provided for each possible tag
  <
    X extends { _tag: string },
    K extends {
      [k in X['_tag']]: (member: Extract<X, { _tag: k }>) => any
    },
  >(
    handlers: K,
  ): (member: X) => ReturnType<K[keyof K]>

  // Partial match, only provide a handler for the desired tag and handle the rest via a fallback handler
  <
    X extends { _tag: string },
    K extends Partial<{
      [k in X['_tag']]: (_: Extract<X, { _tag: k }>) => any
    }>,
    H,
  >(
    partialHandlers: K & {
      [k in X['_tag']]?: (member: Extract<X, { _tag: k }>) => any
    },
    fallback: (_: Exclude<X, { _tag: keyof K }>) => H,
  ): (member: X) => { [k in keyof K]: ReturnType<NonNullable<K[k]>> }[keyof K] | H
}

/**
 * Execute a function for each possible tag of a discriminated union
 *
 * Basically a more concise & expressive version of doing switch(union._tag) {case "blah": ...}
 */
export const matchTag: MatchTag = ((handlers: any, fallback: any) => (member: any) => {
  if (!handlers[member['_tag']]) return fallback(member) as any

  return handlers[member['_tag']](member) as any
}) as any