Yes great explanation - solved my problem. Thanks!

Jeff

On Wednesday, 24 May 2017 16:27:06 UTC-4, John MacFarlane wrote:
>
> +++ Jeff [May 24 17 08:35 ]: 
> >   I am new to Haskell and I am trying to write a new reader for Pandoc. 
> >   There is something strange about manyTill and I'm a bit stuck. 
> >   I have written a minimal code to present the problem. Suppose I want 
> to 
> >   write a parser that succeeds if the string ends in three equal signs 
> >   "===", and return the string preceeding the "===". I have written 
> three 
> >   versions. One with Parsec, and two with Pandoc: 
> >   import Text.Parsec.String (Parser) 
> >   import Text.Parsec (parse, ParseError) 
> >   import Text.Parsec.Combinator (many1, manyTill) 
> >   import Text.Parsec.Char (anyChar, string, noneOf) 
> >   import qualified Text.Pandoc.Builder as B (str) 
> >   import Text.Pandoc.Builder (Inlines) 
> >   import Text.Parsec.Prim (try) 
> >   simpleParse :: Parser a -> String -> Either ParseError a 
> >   simpleParse p = parse p "" 
> >   header' :: Parser String 
> >   header' = manyTill anyChar (string "===") 
> >   header'' :: Parser [Inlines] 
> >   header'' = manyTill (B.str <$> (many1 anyChar)) (string "===") 
> >   header''' :: Parser [Inlines] 
> >   header''' = manyTill (B.str <$> (many1 (noneOf "="))) (string "===") 
> >   I thought header' and header'' are the same parser except the type is 
> a 
> >   bit different. 
>
> There's an important difference.  header' will parse one 
> character at a time (anyChar), each time stopping to see 
> if the end condition (string "===") is met. 
>
> header'' will parse CHUNKS of one or more character at 
> a time (many1 anyChar), only checking after each chunk 
> to see if the end condition is met.  Does that explain it? 
>
>

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