榆樹：填充一個紀錄值列表

我試圖做到：

我有一個模型，是與一些主要的紀錄值對。我想用字符串列表中的值填充這些鍵。

module Main exposing (..) 
import List 
import Html exposing (Html, program, div, text) 

type alias Model = 
    { one: String 
    , two: String 
    , three: String 
    } 

fakeData: List String 
fakeData = ["foo", "bar", "baz", "bad", "baf"] 

populate: Model -> List String -> Model 
populate model data = 
    case List.head data of 
     Just str -> 
      case model.one of 
       "" -> 
        let updatedModel = 
         { model | one = str } 
        in 
         case List.tail data of 
          Just items -> 
           populate updatedModel items 
          Nothing -> 
           model 
       _ -> 
        case model.two of 
         "" -> 
          let updatedModel = 
           { model | two = str } 
          in 
           case List.tail data of 
            Just items -> 
             populate updatedModel items 
            Nothing -> 
             model 
         _ -> 
          case model.three of 
           "" -> 
            let updatedModel = 
             { model | three = str } 
            in 
             case List.tail data of 
              Just items -> 
               populate updatedModel items 
              Nothing -> 
               model 
           _ -> 
            model 
     Nothing -> 
      model 


init: (Model, Cmd Msg) 
init = 
    (populate { one = "", two = "", three = "" } fakeData, Cmd.none) 

type Msg = 
    NoOp 

view: Model -> Html Msg 
view model = 
    div [] 
     [ text (toString model) ] 

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

subscriptions: Model -> Sub Msg 
subscriptions model = 
    Sub.none 

main: Program Never Model Msg 
main = 
    program 
    { init = init 
    , view = view 
    , update = update 
    , subscriptions = subscriptions 
    }

這個程序打印出：

{ one = "foo", two = "bar", three = "baz" }

我想我有一個很難搞清楚如何使這個代碼少重複，更容易推理。如果我在模型中有20個鍵都需要填充，該怎麼辦？上面的代碼會變得非常瘋狂。

來源

2017-07-27 mylvinta

@ chadgilbert的答案是（一如既往）非常好。詳細闡述一下：在Elm中，記錄並非真正用於迭代。通常情況下，當你的代碼在Elm中真的被延長時，這是一個跡象表明其他事情正在發生（也許是另一種類型）。值得看看Elm中的'Dict'類型。在你的模型中使用其中之一可能會讓事情變得更容易。 – wintvelt

您可以在列表，而不是在使用模式匹配：

populate : Model -> List String -> Model 
populate model data = 
    case data of 
     one :: two :: three :: _ -> 
      { model | one = one, two = two, three = three } 

     _ -> 
      model

來源

2017-07-27 18:56:32

哦！這很好，謝謝。 – mylvinta

榆樹：填充一個紀錄值列表

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