haskell - 解析 attoparsec 中不以某些字符结尾的标识符

Question

我一直在编写一个 attoparsec 解析器来解析 Uniform Code for Units of Measure 的内容。调用<ATOM-SYMBOL> 。它被定义为某个类(该类包括所有数字 0-9)中不以数字结尾的最长字符序列。

因此输入 foo27我要消费并返回foo ，对于237bar26我要消费并返回237bar ，对于19我想在不消耗任何东西的情况下失败。

我不知道如何用 takeWhile1 构建这个或takeTill或scan但我可能遗漏了一些明显的东西。

更新:到目前为止，我最好的尝试是设法排除完全是数字的序列

atomSymbol :: Parser Text
atomSymbol = do
               r <- core
               if (P.all (inClass "0-9") . T.unpack $ r)
                 then fail "Expected an atom symbol but all characters were digits."
                 else return r
  where
    core = A.takeWhile1 $ inClass "!#-'*,0-<>-Z\\^-z|~"

我尝试更改它来测试最后一个字符是否是数字，而不是它们是否都是数字，但它似乎不会一次回溯一个字符。

更新2:

整个文件位于 https://github.com/dmcclean/dimensional-attoparsec/blob/master/src/Numeric/Units/Dimensional/Parsing/Attoparsec.hs 。这仅针对 prefixes 构建来自 https://github.com/dmcclean/dimensional 的分支.

Answer 1

您应该重新表述问题并分别处理数字范围 ( 0-9 ) 和非数字字符范围 ( !#-'*,:-<>-Z\\^-z|~ )。感兴趣的句法元素可以描述为

• 可选的数字范围，后跟
• 非数字范围，后跟
• 零个或多个{数字范围后跟一个非数字范围}。

{-# LANGUAGE OverloadedStrings #-}

module Main where

import Control.Applicative ((<|>), many)
import Data.Char (isDigit)

import Data.Attoparsec.Combinator (option)
import Data.Attoparsec.Text (Parser)
import qualified Data.Attoparsec.Text as A
import Data.Text (Text)
import qualified Data.Text as T

atomSymbol :: Parser Text
atomSymbol = f <$> (option "" digitSpan)
               <*> (nonDigitSpan <|> fail errorMsg)
               <*> many (g <$> digitSpan <*> nonDigitSpan)
  where
    nonDigitSpan = A.takeWhile1 $ A.inClass "!#-'*,:-<>-Z\\^-z|~"
    digitSpan    = A.takeWhile1 isDigit
    f x y xss    = T.concat $ x : y : concat xss
    g x y        = [x,y]
    errorMsg     = "Expected an atom symbol but all characters (if any) were digits."

测试

[...] given the input foo27 I want to consume and return foo, for 237bar26 I want to consume and return 237bar, for 19 I want to fail without consuming anything.

λ> A.parseOnly atomSymbol "foo26"
Right "foo"

λ> A.parseOnly atomSymbol "237bar26"
Right "237bar"

λ> A.parseOnly atomSymbol "19"
Left "Failed reading: Expected an atom symbol but all characters (if any) were digits."