我一直在关注这个教程：
http://guide.elm-lang.org/architecture/user_input/forms.html
那里的文字对我有意义,我的问题与它在页面底部列出的练习有关.它问我：
“Add an additional field for age and check that it is a number.”
我对此有困难,因为onInput函数似乎只接受String输入.我觉得奇怪的是没有等效的type =“number”输入.
然而,这是我的尝试不起作用：
import Html exposing (..)
import Html.App as Html
import Html.Attributes exposing (..)
import Html.Events exposing (onInput)
import String exposing (length)
main =
Html.beginnerProgram { model = model,view = view,update = update }
-- MODEL
type alias Model =
{ name : String,password : String,passwordAgain : String,age : Int
}
model : Model
model =
Model "" "" "" 0
-- UPDATE
type Msg
= Name String
| Password String
| PasswordAgain String
| Age Int
update : Msg -> Model -> Model
update msg model =
case msg of
Name name ->
{ model | name = name }
Password password ->
{ model | password = password }
PasswordAgain password ->
{ model | passwordAgain = password }
Age age ->
{ model | age = age }
-- VIEW
view : Model -> Html Msg
view model =
div []
[ input [ type' "text",placeholder "Name",onInput Name ] [],input [ type' "password",placeholder "Password",onInput Password ] [],placeholder "Re-enter Password",onInput PasswordAgain ] [],input [ type' "number",placeholder "Age",onInput Age ] [],viewValidation model
]
viewValidation : Model -> Html msg
viewValidation model =
let
(color,message) =
if model.password /= model.passwordAgain then
("red","Passwords do not match!")
else if length model.password <= 8 then
("red","Password must be more than 8 characters!")
else
("green","OK")
in
div [ style [("color",color)] ] [ text message ]
我得到的错误如下：
-- TYPE MISMATCH ----------------------------------------------------- forms.elm
The argument to function `onInput` is causing a mismatch.
58| onInput Age
^^^
Function `onInput` is expecting the argument to be:
String -> a
But it is:
Int -> Msg
注意：我知道我可以创建Age输入作为另一个文本输入,但练习特别要求我检查它是一个`数字类型.我认为这意味着我应该将它作为Int保存在模型中.
解决方法
来自onInput事件的任何用户输入都是String.
您的模型期望它是一个Int
使用String.toInt从字符串值中解析整数值.
调整更新功能以将类型转换为Int并将类型签名更改为Age String
Age age ->
case String.toInt age of
Ok val ->
{ model | age = val }
-- Notify the user,or simply ignore the value
Err err ->
model
如果Maybe值更适合你,整个语句可以简化为：
Age age ->
{ model | age = Result.toMaybe (String.toInt age) }
