Question

我尝试使用折叠和追加进行字节串操作。请参阅下面的代码。

import qualified Data.ByteString.Char8 as C

selectSource :: [String] -> C.ByteString
selectSource xs = C.append (foldl addSource emptySource xs) (C.pack "<head>")

相应地定义了

addSource和emptySource。括号看起来有点难看，我想在selectSource函数中删除它们

C.append $ foldl addSource emptySource xs $ C.pack "<head>"

但未能这样做并收到错误信息说

Couldn't match expected type ‘C.ByteString -> C.ByteString’
            with actual type ‘C.ByteString’
The first argument of ($) takes one argument,
but its type ‘SourceByteString’ has none
In the second argument of ‘($)’, namely
  ‘foldl addSource emptySource [] $ C.pack "<head>"’

这有效

C.append (foldl addSource emptySource xs) $ C.pack "<head>"

但它仍然有最后一对括号。

Answer 1

使用C.append作为infix operator，您可以不加括号编写此内容。

selectSource :: [String] -> C.ByteString
selectSource xs = foldl addSource emptySource xs `C.append` C.pack "<head>"

由于ByteString拥有mappend = append个Monoid个实例，您可以使用中缀import Data.Monoid selectSource :: [String] -> C.ByteString selectSource xs = foldl addSource emptySource xs <> C.pack "<head>"运算符Monoid更优雅地编写此实例。

"<head>"

如果启用<>，您可以编写ByteString之类的字符串文字，并将其用作ByteString。 fromString的{{1}}酌情pack或packChars（pack Char8为ByteString。这可以摆脱一组括号，这将使你可以在没有任何括号的情况下编写selectSource无点。

{-# LANGUAGE OverloadedStrings #-}

selectSource :: [String] -> C.ByteString
selectSource = flip C.append "<head>" . foldl addSource emptySource

如果您更喜欢flip的操作符，则可以使用OverloadedStrings来编写此点。这需要操作符周围的括号用于操作符部分语法。操作符部分括号不会打扰我，因为我从不嵌套它们。

{-# LANGUAGE OverloadedStrings #-}    
import Data.Monoid

selectSource :: [String] -> C.ByteString
selectSource = (<> "<head>") . foldl addSource emptySource

使用无点定义，我们可以添加其他函数，如C.init，而不会弄乱括号。

selectSource' :: [String] -> C.ByteString
selectSource' = flip C.append "<head>" . C.init . foldl addSource emptySource

selectSource' :: [String] -> C.ByteString
selectSource' = (<> "<head>") . C.init . foldl addSource emptySource

删除haskell语句中的括号

1 个答案: