A ValidationError can be a simple Error or an ErrorList

type YourError
    = ErrorX
    | ErrorY String Int


Error ErrorX


Error (ErrorY "some message" 15)


ErrorList [ ErrorX, ErrorY "some message" 15 ]

Combines two ValidationError

append (Error 1) (Error 2)
    |> Expect.equal (ErrorList [ 1, 2 ])


append (Error 1) (ErrorList [ 2, 3 ])
    |> Expect.equal (ErrorList [ 1, 2, 3 ])


append (ErrorList [ 2, 3 ]) (Error 1)
    |> Expect.equal (ErrorList [ 1, 2, 3 ])


append (ErrorList [ 1, 2 ]) (ErrorList [ 3, 4 ])
    |> Expect.equal (ErrorList [ 1, 2, 3, 4 ])

Converts a ValidationError into a List err

toList (Error 1)
    |> Expect.equal [ 1 ]


toList (ErrorList [ 1, 2, 3 ])
    |> Expect.equal [ 1, 2, 3 ]

Map a function over a ValidationError

errorMap ((+) 1) (Error 1)
    |> Expect.equal (Error 2)


errorMap ((+) 1) (ErrorList [ 1, 2, 3 ])
    |> Expect.equal (ErrorList [ 2, 3, 4 ])

A Validation can be a Success or a Failure

Success "output value"


Failure (Error "not valid")


Failure (ErrorList ["too short", "forbidden character"])

Returns a failed Validation

failure "not valid"
    |> Expect.equal (Failure (Error "not valid"))

Returns a successful Validation

success "output value"
    |> Expect.equal (Success "output value")

Helps creating a basic Validation function

type YourError
    = ErrorX


emptyValidation : String -> Validation YourError String
emptyValidation =
    validation ErrorX String.isEmpty


emptyValidation ""
    |> Expect.equal (Success "")


emptyValidation "notempty"
    |> Expect.equal (Failure (Error ErrorX ))

Map a function over a successful Validation

map ((+) 1) (Success 1)
    |> Expect.equal (Success 2)


map ((+) 1) (Error 1)
    |> Expect.equal (Error 1)

Map a function over a failed Validation

mapError ((+) 1) (Success 1)
    |> Expect.equal (Success 1)


mapError ((+) 1) (Failure (Error 1))
    |> Expect.equal (Failure (Error 2))


mapError ((+) 1) (Failure (ErrorList [ 1, 2 ]))
    |> Expect.equal (Failure (ErrorList [ 2, 3 ]))

Map a function over a failed Validation on the ValidationError

tranform : ValidationError Int -> ValidationError Int
tranform v =
    case v of
        Error n ->
            ErrorList (List.repeat 5 n)


        ErrorList l ->
            ErrorList l


mapValidationError tranform (Success 1)
    |> Expect.equal (Success 1)


mapValidationError tranform (Failure (Error 1))
    |> Expect.equal (Failure (ErrorList [ 1, 1, 1, 1, 1 ]))

Chain together many Validation by binding them. This means we only continue if the validations are successful and we stop at the first Failure

type alias Example =
    { a : String
    , b : String
    }


success Example
    |> andMap (success "valid1")
    |> andMap (success "valid2")
    |> Expect.equal (Success (Example "valid1" "valid2"))


success Example
    |> andMap (failure "error1")
    |> andMap (success "valid2")
    |> Expect.equal (failure "error1")


success Example
    |> andMap (success "valid1")
    |> andMap (failure "error2")
    |> Expect.equal (failure "error2")


success Example
    |> andMap (failure "error1")
    |> andMap (failure "error2")
    |> Expect.equal (failure "error1")

Chain together many Validation by accumulating them. This means we always continue and accumulate all the Failures

type alias Example =
    { a : String
    , b : String
    }


success Example
    |> andMapAcc (success "valid1")
    |> andMapAcc (success "valid2")
    |> Expect.equal (Success (Example "valid1" "valid2"))


success Example
    |> andMapAcc (failure "error1")
    |> andMapAcc (success "valid2")
    |> Expect.equal (failure "error1")


success Example
    |> andMapAcc (success "valid1")
    |> andMapAcc (failure "error2")
    |> Expect.equal (failure "error2")


success Example
    |> andMapAcc (failure "error1")
    |> andMapAcc (failure "error2")
    |> Expect.equal (Failure (ErrorList [ "error2", "error1" ]))