在 Elm 的绑定中是否允许进入惰性？

Question

我有两个功能 A 和 B，可以同时禁用、启用 A、启用 B，但不能同时启用。看完后 Making Impossible States Impossible 我想尝试在类型级别上强制执行此操作。

我正在考虑的解决方案的简化版本如下。

module Main exposing (main)

import Browser
import Html exposing (Html, button, div, text)
import Html.Events exposing (onClick)

type Model
  = NoneEnabled
  | AEnabled
  | BEnabled

init : Model
init = NoneEnabled

type Msg
  = EnableA
  | DisableA
  | EnableB
  | DisableB

view : Model -> Html Msg
view model =
  let -- Buttons to enable and disable features
      buttons =
        div [] [ button [onClick EnableA] [text "Enable A"]
               , button [onClick DisableA] [text "Disable A"]
               , button [onClick EnableB] [text "Enable B"]
               , button [onClick DisableB] [text "Disable B"]
               ]

      -- All possible feature states
      aEnabled  = div [] [text "A enabled"]
      aDisabled = div [] [text "A disabled"]
      bEnabled  = div [] [text "B enabled"]
      bDisabled = div [] [text "B disabled"]
  in case model of
       NoneEnabled ->
         div [] [buttons, aDisabled, bDisabled]
       AEnabled ->
         div [] [buttons, aEnabled, bDisabled]
       BEnabled ->
         div [] [buttons, aDisabled, bEnabled]

update : Msg -> Model -> Model
update msg model =
  case (msg, model) of
    (EnableA, _) ->
      AEnabled
    (EnableB, _) ->
      BEnabled
    (DisableA, AEnabled) ->
      NoneEnabled
    (DisableB, BEnabled) ->
      NoneEnabled
    _ ->
      model

main : Program () Model Msg
main =
  Browser.sandbox { init = init, update = update, view = view }

我的 aEnabled、aDisabled、bEnabled 和 bDisabled 特征在 view 中的计算成本可能很高。无论 case model of 采用哪个分支，它们都会被评估，还是我可以仅依赖被评估的已用功能？

或用更短的例子来表达。

f c x =
  let a = x + 1
      b = x + 2
  in case c of
       True ->
         a
       False ->
         b

f True 0 会在 let 表达式中强制计算 b 吗？

Answer 1

Elm 的 let/in 语句不会延迟计算。你可以用一些Debug.log语句来证明这一点：

f c x =
  let a = Debug.log "a calculated" <| x + 1
      b = Debug.log "b calculated" <| x + 2
  in case c of
       True ->
         a
       False ->
         b

只调用一次 f 会将两条消息都记录到控制台，而不管输入是什么。 Example here.

绕过这个障碍的一种方法是要求 a 和 b 的任意参数，例如单位 ():

f c x =
  let a () = Debug.log "a calculated" <| x + 1
      b () = Debug.log "b calculated" <| x + 2
  in case c of
       True ->
         a ()
       False ->
         b ()

此变体将仅计算函数 a 或 b.

在 Elm 的绑定中是否允许进入惰性？

Is let-in lazy in its bindings in Elm?

lazy-evaluation

elm

let