使单个函数在列表，ByteStrings和Texts（以及其他类似的表示）上工作， [英] Making a single function work on lists, ByteStrings and Texts (and perhaps other similar representations)

问题描述

我正在写一个函数，它可以在一系列任意符号中进行搜索。我想使它足够通用，以便它可以在 Foldable s以及 ByteString s和文本秒。将它推广到 Foldable 很简单。但是如何包含 ByteString s和 Text s？当然我可以将 ByteString 转换成列表，然后调用我的函数，但是我会失去所有优点 ByteString s 。

I'm writing a function that does some searching in a sequence of arbitrary symbols. I'd like to make it generic enough so that it works on lists, Foldables as well on ByteStrings and Texts. Generalizing it to Foldable is simple. But how to include ByteStrings and Texts? Sure I could convert ByteString into a list and then call my function, but I'd lose all the advantages ByteStrings.

有一个具体的例子，假设我们想创建一个直方图函数：

To have a concrete example let's say we want to make a histogram function:

import Control.Monad.State
import qualified Data.Foldable as F
import Data.Map.Strict (Map)
import qualified Data.Map.Strict as Map
import Data.Word
import qualified Data.ByteString as B
import qualified Data.Text as T

type Histogram a = Map a Int

empty :: (Ord a) => Histogram a
empty = Map.empty

histogramStep :: (Ord a) => a -> Histogram a -> Histogram a
histogramStep k = Map.insertWith (+) k 1

histogram :: (Ord a, F.Foldable t) => t a -> Histogram a
histogram = F.foldl (flip histogramStep) empty

但既然 ByteString 也可以是可折叠（它只存储 Word8 s / Char s，而不是任意元素），我坚持创建更多像 的函数，只是使用了不同的类型签名：



But since neither ByteString nor Text can be Foldable (it stores just Word8s/Chars, not arbitrary elements), I'm stuck with creating more functions that look exactly like the one before, just with a different type signatures:

histogramBS :: B.ByteString -> Histogram Word8
histogramBS = B.foldl (flip histogramStep) empty

histogramText :: T.Text -> Histogram Char
histogramText = T.foldl (flip histogramStep) empty

这是一件事不希望像Haskell这样的函数式语言。

This is something one does not expect in a functional language like Haskell.

如何使它成为泛型，要写直方图一次并为所有？

How to make it generic, to write histogram once and for all?

推荐答案

您的解决方案几乎是 ListLike 包。还有一些额外的软件包 listlike-instances ，它为<$ c添加了实例$ c> Text 和 Vector 。

Your solution is pretty much what the ListLike package does. There's also the additional package listlike-instances which adds instances for Text and Vector.

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