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Alaya
Alaya

Reputation: 3387

How to use Parsec to separate a string by a specific string

I am learning parsec, and just encountered the following situation. I want to separate a String into [String] by a specific String; for example, I get "abcSEPdef , and the separator is "SEP", so, after being parsed, I should get a ["abc","def"]

I believe the parser should look like sepBy a_parser (string "SEP"); however, I don't know how the a_parser should be like.

Upvotes: 2

Views: 368

Answers (4)

James Brock
James Brock

Reputation: 3426

The replace-megaparsec package has a sepCap combinator for splitting strings and capturing the separation.

import Replace.Megaparsec
import Text.Megaparsec

parseTest (sepCap (chunk "SEP" :: Parsec Void String String)) "abcSEPdef"

[Left "abc",Right "SEP",Left "def"]

Upvotes: 0

Alaya
Alaya

Reputation: 3387

I finally find a way to incorporate split package into parsec:

module Sep where
import Text.ParserCombinators.Parsec
import qualified Data.List.Split  as DLS
mysep :: String -> Parser [String]
mysep sep = getInput >>= return . DLS.splitOn sep

Upvotes: 1

ErikR
ErikR

Reputation: 52029

Using manyTill a few times will work:

uptoSEP = manyTill anyChar (eof <|> (string "SEP" >> return ()))

splitSEP = manyTill uptoSEP eof

E.g.:

ghci> parseTest splitSEP "abcSEPdefSEPxyz"
["abc","def","xyz"]

You'll want to enable the {-# LANGUAGE NoMonomorphismRestriction #-} pragma.

Upvotes: 3

kyagrd
kyagrd

Reputation: 21

Find a negation of "SEP", and let that parser be parseNonSEP. It is theoretically ensured that there parseNonSEP falls under the category of a regular language, because regular languages are closed under negation, and there should be a straigforward way to implement this.

Then,

sepBy pareseNonSEP (string "SEP")

will do the job.

Well, what I mentioned above is a rather theoretical approach :) More parsec-style way may be to look ahead the list of input tokens without actually consuming the input and/or use backtracking such as try, notFollowedBy, lookAhead.

See

http://hackage.haskell.org/package/parsec-3.1.9/docs/Text-Parsec-Combinator.html

Upvotes: 1

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