A data type like Either but with an accumulating Applicative instance.
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The Validation data type is isomorphic to Either, but has an instance
of Applicative that accumulates on the error side. That is to say, if two
(or more) errors are encountered, they are appended using a Semigroup
operation.
As a consequence of this Applicative instance, there is no corresponding
Bind or Monad instance. Validation is an example of, "An applicative
functor that is not a monad."
The library provides:
- Classy optics (
GetValidation,HasValidation,ReviewValidation,AsValidation) following the conventions ofmakeClassyandmakeClassyPrismsfromlens. - Polymorphic prisms (
__Failure,__Success) for type-changing operations. - Isomorphisms to
Eitherand(Bool, a).
ValidationMonadT err m a is a monad transformer wrapping m (Validation err a).
Unlike Validation, it has short-circuiting Applicative, Bind, Monad,
and MonadError instances.
ValidationMonad err a is a type alias for ValidationMonadT err Identity a.
The library provides four validator newtypes, each wrapping a validation function with a different type parameter order to enable different class instances:
| Type | Wraps | Key instances |
|---|---|---|
Validator x err a |
x -> Validation err a |
Bifunctor, accumulating Applicative, Either-like Alt |
ValidatorProfunctor err x a |
x -> Validation err a |
Profunctor, accumulating Applicative |
ValidatorMonadT x err f a |
x -> ValidationMonadT err f a |
Monad, MonadTrans, BindTrans |
ValidatorMonadProfunctorT err f x a |
x -> ValidationMonadT err f a |
Profunctor, Monad, Category, Arrow |
All four are isomorphic and have cross-type optics instances for converting between them.
Validator is the odd one out in its Alt instance. For Validation and the
other three validators, <!> accumulates errors when both sides fail. For
Validator, <!> behaves like Either: the first success wins, otherwise
the second failure is returned, and no Semigroup constraint is needed. For
this reason Validator has no Plus or Alternative instance. Its <> still
accumulates errors, so use <> when you want accumulation.