CODEWITHSUNDEEP
haskellparsec
Alaya
Alaya

Reputation: 3387

How to implement function trim using Parsec in haskell?

I am learning Parsec and want to practice it by implementing function trim. Here is my code:

module Trim where
import Text.ParserCombinators.Parsec hiding(spaces)
trim = reverse.trimFront.reverse.trimFront
trimFront :: String->String
trimFront = readExpr trimParser
readExpr :: Parser String->String->String
readExpr parser input = case parse parser "trim" input of
                             Left err -> error $ show err
                             Right val -> val
spaces = many space
trimParser :: Parser String
trimParser = spaces >> many anyChar

my question is, How could I implement trim in function trimParser directly without having to implement trimFront first?

Upvotes: 2

Views: 509

Answers (1)

dfeuer
dfeuer

Reputation: 48611

To trim the spaces from both sides of a string:

trim :: String -> String
trim = dropWhileEnd isSpace . dropWhile isSpace

Note that you may be better off using the following implementation of dropWhileEnd instead of the one from Data.List, depending on the situation:

dropWhileEnd :: (a -> Bool) -> [a] -> [a]
dropWhileEnd p = foldr
  (\x xs -> if null xs && p x then [] else x : xs) []

Upvotes: 2

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