@http4t/result

A standard way of representing success/failure results, where in the case of failure we might want to flag issues with multiple elements in a complex data structure.

Basics

A success looks like:

const result = success("this value was ok!");
// -> {value: "this value was ok!"}

A failure looks like:

type Thing = {
  name: string;
  price: number;
}

function validate(value: any) : Result<Thing> {
  const problems = [];

  if(typeof value.name !== 'string')
    problems.push(problem("expected a string", ["name"]))

  if(typeof value.price !== 'number')
    problems.push(problem("expected a number", ["price"]))
  
  if(problems.length > 0) return failure(problems);

  return success(value);
}

validate({});

// -> {problems: [
//          {message: "expected a string", path: ["name"]},
//          {message: "expected a number", path: ["price"]}
//     ]}

Paths

The path in each problem is an array of string | number, which is a subset of jsonpath- just object keys and array indexes.

In the following data structure the value at path ["a", "b", 0, "c"] is 1:

{a: {
  b: [
    {c:1}, 
    {c:2}
  ]
}}

The equivalent json path of ["a", "b", 0, "c"] would be $.a.b.[0].c.

ResultError

Also provided is an error class that plays nicely with test runners and ides.

The error class will calculate an error.expected which is a copy of actual, but with the faulty values replaced with the message of any problems.

const actual = {ok: "ok", wrong: "wrong"};
const error = new ResultError(actual, failure("some error message", ["wrong"]));

console.log(error.expected);
// --> {ok: "ok", wrong: "some error message"}