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（==“”）在Haskell中是什么意思？

[英]What does (== “ ”) mean, in Haskell?

原文 2013-02-13 23:48:20 5 4 haskell/ syntax/ boolean

To the vultures who might say "Look it up in your textbook", or "Hoogle it", I did. 对于那些可能会说“在您的教科书中查找”或“与它联系”的秃鹰，我做到了。

I came across the statement 我碰到了这个声明

recipe = (== "000001")

It looks like some sort of boolean to me but I'm not sure. 对我来说，它看起来像是布尔值，但我不确定。 I've tried testing it in different ways in GHCi but I couldn't figure out anything that works. 我尝试在GHCi中以不同的方式对其进行测试，但我无法找出任何可行的方法。 Can someone explain what it means, and this question will be a result the next time someone Googles Haskell (==" ") 有人可以解释它的意思吗，这个问题将在下次有人Googles Haskell（==“”）时产生

4 个解决方案

It's a section . 这是一节。 It's equivalent to recipe = \\x -> x == "000001" (which in turn is the same as recipe x = x == "000001" ). 它等效于recipe = \\x -> x == "000001" （又与recipe x = x == "000001" ）。

You can use GHCI to figure this one out. 您可以使用GHCI来解决这一问题。

In GHCI , put in let recipe = (== "000001") . 在GHCI ，放入let recipe = (== "000001") 。 Now we can see how it works. 现在我们可以看到它是如何工作的。 Try :t recipe to see what the type is. 尝试:t recipe ，看看类型是什么。 That returns recipe :: [Char] -> Bool , so it looks like this is a function that takes an list of Char s (a String ) and returns a Bool . 这将返回recipe :: [Char] -> Bool ，因此看起来这是一个接受Char列表（ String ）并返回Bool的函数。

If you test it, you'll find it returns False for any input except "000001" . 如果您对其进行测试，则会发现它对除"000001"以外的所有输入都返回False 。

Since == is an operator, you can partially apply it to one argument, and it will return a function that takes the other argument and returns the result. 由于==是一个运算符，因此您可以将其部分地应用于一个参数，它将返回一个接受另一个参数并返回结果的函数。 So here == "000001" returns a function that takes one argument to fill in the other side of the == and returns the result. 因此，这里== "000001"返回一个函数，该函数使用一个参数填充==的另一端并返回结果。

Edit: If the definition were recipe = ((==) "000001") this explanation would be right. 编辑：如果定义是recipe = ((==) "000001")此解释正确。

To understand this, you should look up partial application . 要了解这一点，您应该查找部分应用程序。 The type of the == function is a -> a -> Bool , a function that takes two arguments of the same type and returns a Bool . ==函数的类型是a -> a -> Bool ，该函数接受两个相同类型的参数并返回Bool 。

But it's also a function of type a -> (a -> Bool) , that takes one argument of type a and returns a new function with the signature a -> Bool . 但这也是a a -> (a -> Bool)类型a函数，它接受a类型a参数，并返回一个带有a -> Bool签名a -> Bool新函数。 That's what's happening here. 这就是这里发生的事情。 We've supplied one argument to == , so it returned a new function of type a -> Bool , or [Char] -> Bool in this particular case. 我们为==提供了一个参数，因此在这种特殊情况下，它返回了a -> Bool或[Char] -> Bool类型的新函数。

For binary operator @ the expression (@ x) would mean (\\y -> y @ x) . 对于二元运算符@ ，表达式(@ x)表示(\\y -> y @ x) 。

In your case it will be (\\y -> y == "000001") ie. 在您的情况下，它将是(\\y -> y == "000001")即。 function that takes String and says if it is equal to "000001" . 接受String并说是否等于"000001"函数。

(== arg) or (arg ==) is an operator section (it works for other operators as well - not just == ). (== arg)或(arg ==)是一个运算符部分（它也适用于其他运算符-不仅仅是== ）。 What it does is to partially apply the operator to the given operand. 它的作用是将运算符部分地应用于给定的操作数。 So (== "foo") is the same as \\x -> x == "foo" . 因此(== "foo")与\\x -> x == "foo" 。