表达式中的语法

表达式中的语法 - Haskell(syntax in expression - Haskell)

我是Haskell的新手!! 我写了这段代码：

import Data.List inputIndex :: [String] -> [String] -> Bool inputIndex listx input = and [x `elem` listx |x <- input] inputIndex = if inputIndex == true then putStrLn ("ok")

没有if语句它工作正常但是当我把if语句显示时出现以下错误：

表达式中的语法错误（意外的`}'，可能是由于布局错误）

我在这做错了什么？

谢谢

Im new to Haskell!! I wrote this code:

import Data.List inputIndex :: [String] -> [String] -> Bool inputIndex listx input = and [x `elem` listx |x <- input] inputIndex = if inputIndex == true then putStrLn ("ok")

It works fine without the if statement but when I put the if statement the following error is shown:

Syntax error in expression (unexpected `}', possibly due to bad layout)

What am I doing wrong here?

Thanks

最满意答案

这里有几件事是错的：

你需要一个else子句。 True必须大写。 inputIndex必须总是带两个参数（现在它不会，在最后一种情况下）。

我猜你想要这样的东西......

inputIndex :: [String] -> [String] -> IO () inputIndex listx input = if inputIndex' listx input then putStrLn ("ok") else putStrLn ("not ok") where inputIndex' :: [String] -> [String] -> Bool inputIndex' listx input = and [x `elem` listx |x <- input]

（这里我通过附加一个素数/撇号来定义一个名称几乎相同的新函数。通过在where子句中定义它，它只对外部inputIndex函数可见。你可以将它称为辅助函数，如果你我也可以选择一个完全不同的名字，但我没有创意。）

您还可以将其浓缩为以下（这也更通用）：

allPresent :: (Eq t) => [t] -> [t] -> IO () allPresent xs ys = putStrLn (if and [y `elem` xs | y <- ys] then "ok" else "not ok")

A couple of things are wrong here:

You will need an else clause. True must be capitalized. inputIndex must always take two arguments (right now it does not, in the last case).

I guess you want something like this...

inputIndex :: [String] -> [String] -> IO () inputIndex listx input = if inputIndex' listx input then putStrLn ("ok") else putStrLn ("not ok") where inputIndex' :: [String] -> [String] -> Bool inputIndex' listx input = and [x `elem` listx |x <- input]

(Here I defined a new function with a near-identical name, by appending a prime/apostrophe. By defining it in the where clause, it is only visible to the outer inputIndex function. You can call this a helper-function, if you will. I could also have chosen a completely different name, but I'm uncreative.)

You could also condense this to the following (which is also more general):

allPresent :: (Eq t) => [t] -> [t] -> IO () allPresent xs ys = putStrLn (if and [y `elem` xs | y <- ys] then "ok" else "not ok")