elm - 为什么 Debug.log 在 Elm 中以相反的顺序打印出来？

Question

考虑这个程序:

import Graphics.Element exposing (..)
import Debug


main : Element
main =
  let
    one = Debug.log "one" 1
    two = Debug.log "two" 2
    three = Debug.log "three" 3
  in
    show "Hello"

它将以下内容打印到浏览器的控制台:

three: 3
two: 2
one: 1

为什么顺序颠倒了？

Answer 1

main =
  let
    one = Debug.log "one" 1
    two = Debug.log "two" 2
    three = Debug.log "three" 3
  in
    show "Hello"

实际上被编译为

var main = function () {
  var three = A2($Debug.log,
  "three",
  3);
  var two = A2($Debug.log,
  "two",
  2);
  var one = A2($Debug.log,
  "one",
  1);
  return $Graphics$Element.show("Hello");
}();

请注意订单似乎是如何翻转的。如果我们在 let 绑定(bind)中引入另一个依赖于其他东西的值，则会发生以下情况:

 main =
   let
     one = Debug.log "one" 1
     two = Debug.log "two" 2
     three = Debug.log "three" 3
     four = Debug.log "four" three + one
   in
     show "Hello"

变成

var main = function () {
  var three = A2($Debug.log,
  "three",
  3);
  var two = A2($Debug.log,
  "two",
  2);
  var one = A2($Debug.log,
  "one",
  1);
  var four = A2($Debug.log,
  "four",
  three) + one;
  return $Graphics$Element.show("Hello");
}();

因此，它的长处和短处是不依赖于同一范围内的另一个值的值自下而上处理。当一个值依赖于同一作用域内的另一个值时，单独处理，放在最底层。

这是一个实现细节。