function - 演算子の部分適用

Question

リストを返すために文字の末尾にスペースを追加したい場合、引数を渡さない場合、部分適用でこれをどのように達成しますか?

また、タイプはありますか？

space :: Char -> [Char]

++ および : 演算子を使用して「解析エラー」が発生したため、最後にスペースを追加するのに問題があります。

私がこれまでに持っているものは次のとおりです。

space :: Char -> [Char]
space = ++ ' '

どんな助けでも大歓迎です！ありがとう

Answer 1

Doing what you want is so common in Haskell it's got its own syntax, but being Haskell, it's extraordinarily lightweight. For example, this works:

space :: Char -> [Char]
space = (:" ")

so you weren't far off a correct solution. ([Char] is the same as String. " " is the string containing the character ' '.) Let's look at using a similar function first to get the hang of it. There's a function in a library called equalFilePath :: FilePath -> FilePath -> Bool, which is used to test whether two filenames or folder names represent the same thing. (This solves the problem that on unix, mydir isn't the same as MyDir, but on Windows it is.) Perhaps I want to check a list to see if it's got the file I want:

isMyBestFile :: FilePath -> Bool
isMyBestFile fp = equalFilePath "MyBestFile.txt" fp

but since functions gobble their first argument first, then return a new function to gobble the next, etc, I can write that shorter as

isMyBestFile = equalFilePath "MyBestFile.txt"

This works because equalFilePath "MyBestFile.txt" is itself a function that takes one argument: it's type is FilePath -> Bool. This is partial application, and it's super-useful. Maybe I don't want to bother writing a seperate isMyBestFile function, but want to check whether any of my list has it:

hasMyBestFile :: [FilePath] -> Bool
hasMyBestFile fps = any (equalFilePath "MyBestFile.txt") fps

or just the partially applied version again:

hasMyBestFile = any (equalFilePath "MyBestFile.txt") 

Notice how I need to put brackets round equalFilePath "MyBestFile.txt", because if I wrote any equalFilePath "MyBestFile.txt", then filter would try and use just equalFilePath without the "MyBestFile.txt", because functions gobble their first argument first. any :: (a -> Bool) -> [a] -> Bool

Now some functions are infix operators - taking their arguments from before and after, like == or <. In Haskell these are just regular functions, not hard-wired into the compiler (but have precedence and associativity rules specified). What if I was a unix user who never heard of equalFilePath and didn't care about the portability problem it solves, then I would probably want to do

hasMyBestFile = any ("MyBestFile.txt" ==)

and it would work, just the same, because == is a regular function. When you do that with an operator function, it's called an operator section.

It can work at the front or the back:

hasMyBestFile = any (== "MyBestFile.txt")

and you can do it with any operator you like:

hassmalls = any (< 5)

and a handy operator for lists is :. : takes an element on the left and a list on the right, making a new list of the two after each other, so 'Y':"es" gives you "Yes". (Secretly, "Yes" is actually just shorthand for 'Y':'e':'s':[] because : is a constructor/elemental-combiner-of-values, but that's not relevant here.) Using : we can define

space c = c:" "

and we can get rid of the c as usual

space = (:" ")

which hopefully make more sense to you now.

Answer 2

ここで必要なのは、演算子セクションです。そのためには、アプリケーションを括弧で囲む必要があります。

space = (: " ")

これは糖衣構文です

space = (\x -> x : " ")

(++)最初の引数として文字列を想定しているため、ここでは機能しません。比較してください。

(:)  :: a -> [a] -> [a]
(++) :: [a] -> [a] -> [a]