Haskell:在元组列表上使用"find"功能 [英] Haskell : using the `find` function on a list of tuples
问题描述
我想知道是否有人可以帮助我.假设我有一个(String,Int)类型的元组，以及这样的元组[("Strength",12),("Stamina",60),("Health",100)]的列表.
I was wondering if anyone could help me. Suppose I have a tuple of type (String,Int), and a list of such tuples [("Strength",12),("Stamina",60),("Health",100)] .
如果我实际上不知道元组在列表中的顺序，而只是某个包含字符串的特定元组<，则如何使用函数find提取元组Int的值("Stamina",60) c5>存在吗?
How can I use the function find to extract the Int value of the tuple ("Stamina",60) if I don't actually know what order the tuples are inside the list but only that a certain tuple containing the string "Stamina" exists?
我尝试过
value = snd ( find ("Stamina", _ ) stats )
其中stats是元组列表，而value定义为
where stats is the list of tuples and value is defined as
value :: a -> Int
..但是它不起作用:/那么有什么主意吗?我专门打算使用find.
..but it doesn't work :/ So any ideas? I'm specifically meant to use find.
推荐答案
似乎您正在尝试使用模式作为find的参数.不幸的是，那是行不通的.您只能在几个地方进行模式匹配，例如后卫和case表达式.
It seems like you're trying to use a pattern as an argument to find. Unfortunately, that won't work. You can only do pattern matching in a few places, such as guards and case expressions.
正如Jubobs在他的评论中所说，在这种情况下使用Data.List.lookup是理想的选择，但是使用Data.List.find绝对也是可行的.
As Jubobs has said in his comment, it's ideal to use Data.List.lookup in this case, but using Data.List.find is definitely possible as well.
如果查看Data.List.find的类型，您会发现它看起来像这样:
If you take a look at the type of Data.List.find, you'll see that it looks like this:
find :: Foldable t => (a -> Bool) -> t a -> Maybe a
这告诉您两个重要的事情:
This tells you two important things:
-
第一个参数必须是谓词函数，即一个函数，当它的参数为您要查找的值时返回True，否则返回False
The first argument has to be a predicate function, i.e. a function that returns True when it's argument is of the value you are looking for and returns False otherwise
该函数返回Maybe a，这意味着它可能会返回Nothing，您必须对此进行处理.
The function returns a Maybe a, which means it may return Nothing and you have to deal with this.
创建谓词功能非常简单.您只需要使用==运算符来测试元组的第一个值的东西，也许是像这样的lambda函数:
Creating the predicate function is pretty easy. You just need something that tests the first value of the tuple using the == operator, perhaps a lambda function such as this:
\(x, _) -> x == "Stamina"
现在您可以像这样调用find:
And now you can call find like this:
find (\(x, _) -> x == "Stamina") stats
或者，您可以创建一个通用函数来将元组的第一个元素与已知值进行比较，例如:
Alternatively, you could create a general function for comparing the first element of a tuple to a known value, such as the following:
matchFirst x (y, _) = x == y
然后将matchFirst函数用作find的参数:
And later on use the matchFirst function as an argument to find:
find (matchFirst "Stamina") stats
现在让我们看看我的第二点:您如何处理find找不到任何东西?
Now let's take a look at my second point: how do you deal with find not finding anything?
如果您完全确定它会一直成功，则可以使用Data.Maybe.fromJust从Maybe中提取元组，如下所示:
If you are absolutely sure that it will always succeed, you can simply use Data.Maybe.fromJust to extract the tuple from the Maybe like this:
value = snd $ fromJust $ find (matchFirst "Stamina") stats
否则，如果查找实际上可能失败，则可以执行许多操作.例如，您可以在Data.Maybe.fromMaybe中使用合理的默认值，也可以将value更改为Maybe Int类型.
Otherwise, if the lookup can actually fail, there are many things you can do. You can, for example, use a sensible default value with Data.Maybe.fromMaybe or you can change your value to have the type Maybe Int.
最后，最后一件事:您说过value具有类型a -> Int，即，一个接受任何内容并返回Int的函数.实际上并非如此.相反，它只是一个值.
Finally, one last thing: you've said that value has the type a -> Int, i.e. a function that takes anything and returns an Int. This is not actually the case. Instead, it's just a single value.
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