Represents a validator of the input value. Validation can fail with a list of custom error messages.

Build a custom validator.

intValidator : Validator String InputError Int
intValidator =
    Validator.custom (String.toInt >> Result.fromMaybe [ NaN ])

Run validator.

intValidator : Validator String String Int
intValidator =
    Validator.custom (String.toInt >> Result.fromMaybe [ "Not a number" ])


Validator.run intValidator "42"
--> (Ok 42)


Validator.run intValidator "Fat fingers"
--> (Err [ "Not a number" ])

Create validator that always succeed.

Validator.run (Validator.succeed 3) "blah" --> (Ok 3)

Create validator that always fails.

Validator.run (Validator.fail "Invalid") 123 --> (Err [ "Invalid" ])

Transform validated value with a given function.

Validator.run (Validator.map ((+) 1) (Validator.succeed 2)) Nothing
--> (Ok 3)

Apply a function if both arguments passed validation.

Validator.map2 (\x y -> x + y)
    (Validator.succeed 1)
    (Validator.succeed 2)
    |> (\validator -> Validator.run validator ())
    --> (Ok 3)

Apply wrapped function to another wrapped value.

intValidator : Validator String String Int
intValidator =
    Validator.custom (String.toInt >> Result.fromMaybe [ "Not a number" ])


Validator.run (Validator.succeed ((+) 2) |> Validator.andMap intValidator) "3"
--> (Ok 5)

Chain together many validators.

intValidator
    |> Validator.andThen
        (\x ->
            if x < 0 then
                Validator.fail "Must be a positive number"


            else
                Validator.succeed x
        )
    -- ...

Combine validators with required field.

type alias User =
    { name : String
    , age : Int
    , email : Email
    }


type Email = Email String


parseEmail : String -> Result String Email
parseEmail str =
    if String.contains "@" str then
        Ok (Email str)
    else
        Err "Invalid email"


emailValidator : Validator String String Email
emailValidator =
    Validator.custom (parseEmail >> Result.mapError List.singleton)


intValidator : Validator String String Int
intValidator =
    Validator.custom (String.toInt >> Result.fromMaybe [ "Not a number" ])


type alias FormValues r =
    { r
        | name : String
        , age : String
        , email : String
    }


userValidator : Validator (FormValues r) String User
userValidator =
    Validator.succeed User
        |> Validator.required .name String.isEmpty "Name is required" (Validator.custom Ok)
        |> Validator.required .age String.isEmpty "Age is required" intValidator
        |> Validator.required .email String.isEmpty "Email is required" emailValidator


Validator.run userValidator { name = "John Doe", age = "", email = "test" }
    --> Err [ "Age is required", "Invalid email" ]

Combine validators with optional field.

type alias User =
    { name : String
    , age : Int
    , email : Email
    }


type Email = Email String


parseEmail : String -> Result String Email
parseEmail str =
    if String.contains "@" str then
        Ok (Email str)
    else
        Err "Invalid email"


emailValidator : Validator String String Email
emailValidator =
    Validator.custom (parseEmail >> Result.mapError List.singleton)


intValidator : Validator String String Int
intValidator =
    Validator.custom (String.toInt >> Result.fromMaybe [ "Not a number" ])


type alias FormValues r =
    { r
        | name : String
        , age : String
        , email : String
    }


userValidator : Validator (FormValues r) String User
userValidator =
    Validator.succeed User
        |> Validator.required .name String.isEmpty "Name is required" (Validator.custom Ok)
        |> Validator.optional .age String.isEmpty 10 intValidator
        |> Validator.required .email String.isEmpty "Email is required" emailValidator


Validator.run userValidator { name = "John Doe", age = "", email = "happy@path" }
    --> Ok (User "John Doe" 10 (Email "happy@path"))