榆树签名在"Program Never Model Msg"中是什么意思? [英] what does the elm signature mean in "Program Never Model Msg"?

问题描述

我只是开始使用Elm，并且不了解Haskell及其编译器我正在尝试掌握 Html.program

中的签名的含义

  func:(a->字符串)->字符串-这意味着需要一个函数并返回一个字符串main:程序Never Model Msg-这是什么意思? 

解决方案

Program 是由以下三个类型变量参数化的类型: flags ， model 和 msg. 从不 是不能有任何值的类型(请参阅链接，以获取对它的含义以及与单位类型()的区别)的很好解释.

程序从不模型消息因此是没有任何标志(从不)，模型类型为 Model

的程序的类型code>，并传递类型为 Msg 的消息.

I'm just starting on elm and without understanding Haskell and its compiler I'm trying to grasp what the signature mean in Html.program

func: (a -> String) -> String -- this means expects a function and return a string
main: Program Never Model Msg -- What does this mean?

解决方案

Program is a type parameterized by three type variables: flags, model, and msg. Never is a type that cannot have any value (see the link for a good explanation of what this means and how it differs from the unit type ()).

Program Never Model Msg therefore is the type of a program that doesn't have any flags (Never), has a model of type Model, and passes messages of type Msg.

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