swift-validation

An experimental library for validating just about anything.

Motivation

Inspired by swift-parsing, this library provides a declarative and composable way of validating any type of input into a Validated struct.

⚠️ This library is not meant to be used in production code. It is an exercise to learn more about Swift type system and result builders.

Getting started

Validated

The library exposes a Validated type and some helpers to accumulate validation errors.

enum Validated<Valid> {
 case valid(Valid)
 case invalid([ValidationError])
}

Validator

A Validator is defined by Input associated type representing the type to be validated. A Valid associated type representing the possibly transform validated Input. Finally, a validate function to validate an Input into a Validated<Valid>.

public protocol Validator {
  associatedtype Input
  associatedtype Valid
  
  func validate(_ input: Input) -> Validated<Valid>
}

Usage

The most common way of creating a Validator is through the various builders.

The Validate validator, is an entry point for the ValidateBuilder. It validates an input by running it again various Validators of the matching Input and Valid associated value. It accumulates errors.

Validate {
  Predicate<String> { !$0.isEmpty }
  Predicate<String> { $0 != "bad" }
}

The OneOf validator is an entry point for the OneOfBuilder. It validates an input by running it again various Validators of the matching Input and Valid associated value. It succeeds if one of the validators succeeds and only fails if all of them fail.

OneOf {
  Predicate("a")
  Predicate("b")
  Predicate("c")
}

The ValidateThrough validator is an entry point for the ValidateThroughBuilder. It validates an input by running it again various Validators of the matching Input and Valid associated value. It pipes the Valid value of a previous validator to the next.

enum Enum: String {
  case a, b, c
}

ValidateThrough {
  String?.validator()
  Enum.validator()
}

Playground

The library comes with a playground that you can run to experiment with the different validators.