result.ts

interface Success<a> {
  value: a;
}

interface Failure<e> {
  error: e;
}

type Result<a, e> = Success<a> | Failure<e>;

const unit = <a>(a: a): Success<a> => ({ value: a });
const fail = <e>(e: e): Failure<e> => ({ error: e });

function isSuccess(result: Result<any, any>): result is Success<any>{
  return "value" in result;
}

function isFailure(result: Result<any, any>): result is Failure<any>{
    return "error" in result;
}