Elm Applicative Validation Example

ValidationExample.elm

module Main exposing (..)

import List.Nonempty exposing (..)
import State exposing (State(..))


type alias Error =
    String


mkCharacter : Model -> Result (List Error) Character
mkCharacter m =
    State.state (Just Character)
        |> (\s -> State.andMap s (validateName m))
        |> (\s -> State.andMap s (validateBio m))
        |> (\s -> State.andMap s (validateLore m))
        |> State.run []
        |> (\( c, errors ) -> Result.fromMaybe errors c)


validateName model =
    case model.validName of
        Nothing ->
            State (\errors -> ( always Nothing, errors ++ [ "Name is required" ] ))

        Just validName ->
            State (\errors -> ( Maybe.map ((|>) validName), errors ))


validateBio model =
    case model.validBio of
        Nothing ->
            State (\errors -> ( always Nothing, errors ++ [ "Bio is required" ] ))

        Just validBio ->
            State (\errors -> ( Maybe.map ((|>) validBio), errors ))


validateLore model =
    case model.validLore of
        Nothing ->
            State (\errors -> ( always Nothing, errors ++ [ "Lore is required" ] ))

        Just validLore ->
            State (\errors -> ( Maybe.map ((|>) validLore), errors ))


type alias Model =
    { validName : Maybe String
    , validBio : Maybe (Nonempty String)
    , validLore : Maybe (Nonempty String)
    }


type alias Character =
    { name : String
    , bio : Nonempty String
    , lore : Nonempty String
    }