Validation is a sum type for storing either all validation failures or validation success. Unlike Either, which returns only the first error, Validation accumulates all errors using the Semigroup typeclass.

Usually type variables in Validation e a are used as follows:

• e: is a list or set of failure messages or values of some error data type.
• a: is some domain type denoting successful validation result.

Some typical use-cases:

• Validation [String] User

  • Either list of String error messages or a validated value of a custom User type.

• Validation (NonEmpty UserValidationError) User

  • Similar to previous example, but list of failures guaranteed to be non-empty in case of validation failure, and it stores values of some custom error type.

Predicate on if the given Validation is Failure.

>>> isFailure (Failure 'e')
True
>>> isFailure (Success 'a')
False

Predicate on if the given Validation is Success.

>>> isSuccess (Success 'a')
True
>>> isSuccess (Failure 'e')
False

Transforms the value of the given Validation into x using provided functions that can transform Failure and Success value into the resulting type respectively.

>>> let myValidation = validation (<> " world!") (show . (* 10))
>>> myValidation (Success 100)
"1000"
>>> myValidation (Failure "Hello")
"Hello world!"

Filters out all Failure values into the new list of es from the given list of Validations.

Note that the order is preserved.

>>> failures [Failure "Hello", Success 1, Failure "world", Success 2, Failure "!" ]
["Hello","world","!"]

Filters out all Success values into the new list of as from the given list of Validations.

Note that the order is preserved.

>>> successes [Failure "Hello", Success 1, Failure "world", Success 2, Failure "!" ]
[1,2]

Redistributes the given list of Validations into two lists of es and es, where the first list contains all values of Failures and the second one — Successes correspondingly.

Note that the order is preserved.

>>> partitionValidations [Failure "Hello", Success 1, Failure "world", Success 2, Failure "!" ]
(["Hello","world","!"],[1,2])

Returns the contents of a Failure-value or a default value otherwise.

>>> fromFailure "default" (Failure "failure")
"failure"
>>> fromFailure "default" (Success 1)
"default"

Returns the contents of a Success-value or a default value otherwise.

>>> fromSuccess 42 (Success 1)
1
>>> fromSuccess 42 (Failure "failure")
42

NB Validation is not a monad though

Create a Failure of NonEmpty list with a single given error.

>>> failure "I am a failure"
Failure ("I am a failure" :| [])

Returns a Failure in case of the given predicate is True. Returns Success () otherwise.

>>> let shouldFail = (==) "I am a failure"
>>> failureIf (shouldFail "I am a failure") "I told you so"
Failure ("I told you so" :| [])
>>> failureIf (shouldFail "I am NOT a failure") "okay"
Success ()

Returns a Failure unless the given predicate is True. Returns Success () in case of the predicate is satisfied.

Similar to failureIf with the reversed predicate.

failureUnless p ≡ failureIf (not p)

>>> let shouldFail = (==) "I am a failure"
>>> failureUnless (shouldFail "I am a failure") "doesn't matter"
Success ()
>>> failureUnless (shouldFail "I am NOT a failure") "I told you so"
Failure ("I told you so" :| [])

Combinators

We are providing several functions for better integration with the Either related code in this section.

Transform a Validation into an Either.

>>> validationToEither (Success "whoop")
Right "whoop"

>>> validationToEither (Failure "nahh")
Left "nahh"

Transform an Either into a Validation.

>>> eitherToValidation (Right "whoop")
Success "whoop"

>>> eitherToValidation (Left "nahh")
Failure "nahh"

Validate all given checks in a Foldable. Returns the Success of the start element when all checks are successful.

A basic example of usage could look like this:

> let validatePassword = validateAll
        [ validateEmptyPassword
        , validateShortPassword
        ]

> validateAll "VeryStrongPassword"
Success "VeryStrongPassword"

> validateAll ""
Failure (EmptyPassword :| [ShortPassword])

Applies the given action to Validation if it is Success and returns the result. In case of Failure the default value is returned.

>>> whenSuccess "bar" (Failure "foo") (\a -> "success!" <$ print a)
"bar"

>>> whenSuccess "bar" (Success 42) (\a -> "success!" <$ print a)
42
"success!"

Applies the given action to Validation if it is Failure and returns the result. In case of Success the default value is returned.

>>> whenFailure "bar" (Failure 42) (\a -> "foo" <$ print a)
42
"foo"

>>> whenFailure "bar" (Success 42) (\a -> "foo" <$ print a)
"bar"

Applies given action to the Validation content if it is Success.

Similar to whenSuccess but the default value is ().

>>> whenSuccess_ (Failure "foo") print
>>> whenSuccess_ (Success 42) print
42

Applies given action to the Validation content if it is Failure.

Similar to whenFailure but the default value is ().

>>> whenFailure_ (Success 42) putStrLn
>>> whenFailure_ (Failure "foo") putStrLn
foo

Monadic version of whenSuccess. Applies monadic action to the given Validation in case of Success. Returns the resulting value, or provided default.

>>> whenSuccessM "bar" (pure $ Failure "foo") (\a -> "success!" <$ print a)
"bar"

>>> whenSuccessM "bar" (pure $ Success 42) (\a -> "success!" <$ print a)
42
"success!"

Monadic version of whenFailure. Applies monadic action to the given Validation in case of Failure. Returns the resulting value, or provided default.

>>> whenFailureM "bar" (pure $ Failure 42) (\a -> "foo" <$ print a)
42
"foo"

>>> whenFailureM "bar" (pure $ Success 42) (\a -> "foo" <$ print a)
"bar"

Monadic version of whenSuccess_. Applies monadic action to the given Validation in case of Success. Similar to whenSuccessM but the default is ().

>>> whenSuccessM_ (pure $ Failure "foo") print
>>> whenSuccessM_ (pure $ Success 42) print
42

Monadic version of whenFailure_. Applies monadic action to the given Validation in case of Failure. Similar to whenFailureM but the default is ().

>>> whenFailureM_ (pure $ Success 42) putStrLn
>>> whenFailureM_ (pure $ Failure "foo") putStrLn
foo

Maps Failure of Validation to Just.

>>> failureToMaybe (Failure True)
Just True
>>> failureToMaybe (Success "aba")
Nothing

Maps Success of Validation to Just.

>>> successToMaybe (Failure True)
Nothing
>>> successToMaybe (Success "aba")
Just "aba"

Maps Just to Failure In case of Nothing it wraps the given default value into Success.

>>> maybeToFailure True (Just "aba")
Failure "aba"
>>> maybeToFailure True Nothing
Success True

Maps Just to Success. In case of Nothing it wraps the given default value into Failure

>>> maybeToSuccess True (Just "aba")
Success "aba"
>>> maybeToSuccess True Nothing
Failure True