Elm Form

HTML live forms builders and validation for Elm.

elm package install etaque/elm-form

For when the classical "a message per field" doesn't work well for you, at the price of loosing some typesafety.

Features

Validation API similar to Json.Decode with the standard map, andThen, etc: you either get the desired output value or all field errors
HTML inputs helpers with pre-wired handlers for live validation
Suite of basic validations, with a way to add your own
Unlimited fields, see andMap function (as in Json.Extra)
Nested fields (foo.bar.baz) and lists (todos.1.checked) enabling rich form build

See complete example here (source code).

Basic usage

module Main exposing (..)

import Html exposing (..)
import Html.Attributes exposing (..)
import Html.Events exposing (..)
import Form exposing (Form)
import Form.Validate as Validate exposing (..)
import Form.Input as Input


-- your expected form output

type alias Foo =
    { bar : String
    , baz : Bool
    }


-- Add form to your model and msgs

type alias Model =
    { form : Form () Foo }

type Msg
    = NoOp
    | FormMsg Form.Msg


-- Setup form validation

init : ( Model, Cmd Msg )
init =
    ( { form = Form.initial [] validation }, Cmd.none )

validation : Validation () Foo
validation =
    map2 Foo
        (field "bar" email)
        (field "baz" bool)


-- Forward form msgs to Form.update

update : Msg -> Model -> ( Model, Cmd Msg )
update msg ({ form } as model) =
    case msg of
        NoOp ->
            ( model, Cmd.none )

        FormMsg formMsg ->
            ( { model | form = Form.update validation formMsg form }, Cmd.none )


-- Render form with Input helpers

view : Model -> Html Msg
view { form } =
    Html.map FormMsg (formView form)

formView : Form () Foo -> Html Form.Msg
formView form =
    let
        -- error presenter
        errorFor field =
            case field.liveError of
                Just error ->
                    -- replace toString with your own translations
                    div [ class "error" ] [ text (toString error) ]

                Nothing ->
                    text ""

        -- fields states
        bar =
            Form.getFieldAsString "bar" form

        baz =
            Form.getFieldAsBool "baz" form
    in
        div []
            [ label [] [ text "Bar" ]
            , Input.textInput bar []
            , errorFor bar
            , label []
                [ Input.checkboxInput baz []
                , text "Baz"
                ]
            , errorFor baz
            , button
                [ onClick Form.Submit ]
                [ text "Submit" ]
            ]


app =
    Html.program
        { init = init
        , update = update
        , view = view
        , subscriptions = \_ -> Sub.none
        }

Advanced usage

Custom inputs

For rendering, Form.getFieldAsString/Bool provides a FieldState record with all required fields (see package doc).
For event handling, see all field related messages in Form.Msg type.

Overall, having a look at current helpers source code should give you a good idea of the thing.

Incremental validation

Similar to what Json.Extra provides. Use Form.apply, or the |: infix version from infix package:

Form.succeed Player
    |> andMap (field "email" (string |> andThen email))
    |> andMap (field "power" int)

Nested records

Validation:

validation =
    map2 Player
        (field "email" (string |> andThen email))
        (field "power" (int |> andThen (minInt 0)))
        (field "options"
            (map2 Options
                (field "foo" string)
                (field "bar" string)
            )
        )

View:

Input.textInput (Form.getFieldAsString "options.foo" form) []

Dynamic lists

-- model
type alias TodoList =
    { title : String
    , items : List String
    }

-- validation
validation : Validation () Issue
validation =
    map2 TodoList
        (field "title" string)
        (field "items" (list string))

-- view
formView : Form () Issue -> Html Form.Msg
formView form =
    div
        [ class "todo-list" ]
        [ Input.textInput
            (Form.getFieldAsString "title" form)
            [ placeholder "Title" ]
        , div [ class "items" ] <|
            List.map
                (itemView form)
                (Form.getListIndexes "items" form)
        , button
            [ class "add"
            , onClick (Form.Append "items")
            ]
            [ text "Add" ]
        ]

itemView : Form () Issue -> Int -> Html Form.Msg
itemView form i =
    div
        [ class "item" ]
        [ Input.textInput
            (Form.getFieldAsString ("items." ++ (toString i)) form)
            []
        , a
            [ class "remove"
            , onClick (Form.RemoveItem "items" i)
            ]
            [ text "Remove" ]
        ]

Initial values and reset

At form initialization:

import Form.Field as Field


initialFields : List ( String, Field )
initialFields =
    [ ( "power", Field.string "10" )
    , ( "options"
      , Field.group
            [ ( "foo", Field.string "blah" )
            , ( "bar", Field.string "meh" )
            ]
      )
    ]


initialForm : Form
initialForm =
    Form.initial initialFields validation

See Form.Field type for more options.

On demand:

button [ onClick (Form.Reset initialFields) ] [ text "Reset" ]

Note: To have programmatically control over any input[type=text]/textarea value, like reseting or changing the value, you must set the value attribute with Maybe.withDefault "" state.value, as seen here. There's a downside of doing this: if the user types too fast, the caret can go crazy.

More info: evancz/elm-html#81 (comment)

Custom errors

type LocalError = Fatal | NotSoBad

validation : Validation LocalError Foo
validation =
    (field "foo" (string |> customError Fatal))

-- creates `Form.Error.CustomError Fatal`

Async validation

This package doesn't provide anything special for async validation, but doesn't prevent you to do that neither. As field values are accessible from update with Form.getStringAt/getBoolAt, you can process them as you need, trigger effects like an HTTP request, and then add any errors to the view by yourself.

Another way would be to enable dynamic validation reload, to make it dependant of an effect, as it's part of the form state. Please ping me if this feature would be useful to you.

dynajoe/elm-form