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https://github.com/etaque/elm-form

Dynamic forms handling in Elm
https://github.com/etaque/elm-form

elm form form-builder form-validation

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Dynamic forms handling in Elm

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# Elm Form



HTML live form builders and validation for Elm. [![Build Status](https://travis-ci.org/etaque/elm-form.svg?branch=master)](https://travis-ci.org/etaque/elm-form)



    elm package install etaque/elm-form



For when the classical "a message per field" doesn't work well for you, at the price of losing some type safety (field names are made of strings, see [#97](https://github.com/etaque/elm-form/issues/97)).



## Support



We have a dedicated channel in [Elm slack](https://elmlang.herokuapp.com/), join us in `#elm-form` for any question, support or issues coordination.



## 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](http://etaque.github.io/elm-form/example/) ([source code](https://github.com/etaque/elm-form/tree/master/example)).



## Basic usage



See the [example validation test suite](https://github.com/etaque/elm-form/blob/master/tests/ValidationTests.elm)

and [test helper function docs](http://package.elm-lang.org/packages/etaque/elm-form/latest/Form-Test)

for how to test-drive validations.



```elm

module Main exposing (Foo, Model, Msg(..), app, formView, init, update, validate, view)



import Browser

import Form exposing (Form)

import Form.Input as Input

import Form.Validate as Validate exposing (..)

import Html exposing (..)

import Html.Attributes exposing (..)

import Html.Events exposing (..)



-- 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

init =

    { form = Form.initial [] validate }



validate : Validation () Foo

validate =

    succeed Foo

        |> andMap (field "bar" email)

        |> andMap (field "baz" bool)



-- Forward form msgs to Form.update



update : Msg -> Model -> Model

update msg ({ form } as model) =

    case msg of

        NoOp ->

            model



        FormMsg formMsg ->

            { model | form = Form.update validate formMsg form }



-- 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 (Debug.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 =

    Browser.sandbox

        { init = init

        , update = update

        , view = view

        }



```



## 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](https://github.com/etaque/elm-form/blob/master/src/Form/Input.elm) should give you a good idea of the thing.



### Incremental validation



Similar to what Json.Extra provides you can also use `Form.andMap`



```elm

Form.succeed Player

    |> andMap (field "email" (string |> andThen email))

    |> andMap (field "power" int)

```



### Nested records



* Validation:



```elm

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:



```elm

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

```



### Dynamic lists



```elm

-- 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." ++ (String.fromInt i)) form)

            []

        , a

            [ class "remove"

            , onClick (Form.RemoveItem "items" i)

            ]

            [ text "Remove" ]

        ]

```



### Initial values and reset



* At form initialization:



```elm

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:



```elm

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

```



*Note:* To have programmatic 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](https://github.com/etaque/elm-form/pull/57/files#diff-bfb877e82b2c89b329fcda943a258611R50). There's a downside of doing this: if the user types too fast, the caret can go crazy.



More info: https://github.com/evancz/elm-html/pull/81#issuecomment-145676200



### Custom errors



```elm

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 either. 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.