什么= =>标志在Haskell中的意思？ [英] What does the => sign mean in Haskell?

问题描述

出于某种原因，我无法在任何地方找到答案。我试着用谷歌搜索Haskell等号箭头，我没有得到任何结果。假设我们有以下功能：

  sendMessage :: MonadM e m =>消息 - > m（）
 sendMessage message = do 
 mClient<  -  getMessageClient 
 liftIO $ send mClient message 
  

e和m在哪里被使用？它们被传递给Message对象（函数？）然后作为单一类型输出，m（）？

我认为这不会有帮助，因为我

解决方案

首先：如果你想知道这样和那样的操作符不要问StackOverflow，问问 Hayoo ！

但事实上Hayoo对于 => 没有用处，因为与Haskell中的其他几乎不同，这是内置语法，而不是

您的签名

  sendMessage： ：MonadM em =>消息 - > m（）
  

表示如下： sendMessage 是消息 - > m（），其中 m 可以是任何具有 MonadM 类型的类。



这可能对你没有多大帮助，因为实际上 MonadM 是一个相当复杂的类型类。最好考虑一个更简单的例子：



  sum :: Num n => [n]  - > n 
  

这意味着：和的类型是 [n] - > n ，其中 n 可以是任何数字类型，它是 Num 类，即类型支持减法，乘法，明显 addition 等。实际上，语法是


  sum ::∀n的简写。 Num n => [n]  - > n 
  

表示所有 类型 n 符合约束 Num n ，函数 sum 具有签名 [n] - > n 。

您可以使用任何具体的数字类型实例化这样的多态函数：例如

  sum :: [Int]  - >在你的例子中，你可能将它实例化为类似于
的
 
 
  sendMessage :: Message  - > MessageT IO（）
  
 
For some reason I can't find the answer to this anywhere. I tried Googling "Haskell equal sign arrow" and I'm not getting any results. Let's say we have the following function:
sendMessage :: MonadM e m => Message -> m ()
sendMessage message = do
    mClient  <- getMessageClient
    liftIO $ send mClient message
Where exactly are e and m getting used? Are they being passed into the Message object (function?) and then outputted as a single type, m ()?

I don't think it helps that I'm very new to Haskell, but any help is appreciated here.
解决方案
First off: if you want to know what such and such operator does, don't ask StackOverflow, ask Hayoo!

But in fact Hayoo is no use for => in particular because, unlike almost everything else in Haskell, this is built-in syntax and not an operator that's defined in some library.

Your signature
sendMessage :: MonadM e m => Message -> m ()
means the following: the type of sendMessage is Message -> m (), where m can be any monad that has an instance of the MonadM type class.

That probably doesn't help you much, because in fact MonadM is a rather involved type class. Better consider a simpler example:
sum :: Num n => [n] -> n
This means: the type of sum is [n] -> n, where n can be any number type that's an instance of the Num class, i.e. the class of types supporting subtraction, multiplication, obviously addition etc.. Actually the syntax is shorthand for
sum :: ∀ n . Num n => [n] -> n
meaning that for all types n which fulfill the constraint Num n, the function sum has the signature [n] -> n.

You can instantiate such a polymorphic function with any concrete number type: e.g.
sum :: [Int] -> Int
In your example you'd probably instantiate it to something like
sendMessage :: Message -> MessageT IO ()


                        
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