Haskell-搜索对列表 [英] Haskell - Searching through a list of pairs

问题描述

我正在尝试制作某种东西，以便它需要一本书和一个字符串，然后返回与该书中的字符串相关联的值...所以到目前为止，我是这样的:

I'm trying to make something so that it takes a Book and a string and then returns the value that's associated with the string in the book... so what I have so far is:

data Answer = T | F
    deriving (Eq,  Show, Ord)

type Book = [(String, Answer)] 

testBook :: Book
testBook = [("aT", T), ("bF", F)]

我想这样做，所以我说:

and I want to do it so that let's say i put:

test testBook "aT"

^答案将传给T.

我正在做类似的事情:

test::Book->String->Answer
test a b = [x | (y, x) <- a, y == b] 

但是我知道那完全不合时宜.我如何比较String和Book中的内容?看起来很简单，但是Haskell的语法确实很难习惯

but i know that's completely off.. how can I compare the String to what's in the Book? it seems simple but the syntax for Haskell is really hard to get used to

推荐答案

您的代码在正确的轨道上.现在，您将获得无论值为b的答案的列表.您需要添加一个条件，以检查元组的第一项是否等于b；为此，您必须给它起一个名字而不是使用_.

Your code is on the right track. Right now, you get a list of answers regardless of the value of b. You need to add a condition that checks whether the first item of the tuple is equal to b; to do this, you will have to give it a name rather than using _.

您可以使用逗号和表达式将条件添加到列表理解:

You can add a condition to a list comprehension by using a comma and an expression:

[x | x <- a, x > 10]

例如，

将从a中获取大于10的所有值.

will get you all the values from a greater than 10, for example.

我会让你想出如何自己提出建议的方法.

I will let you figure out how to put my advice together yourself.

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