simple question about using manyTill

simple question about using manyTill

[Jeff]

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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. But I'm wrong:

*Main> simpleParse header' "a==="
Right "a"
*Main> simpleParse header'' "a==="
Left (line 1, column 5):
unexpected end of input
expecting "==="
*Main> simpleParse header''' "a==="
Right [Many {unMany = fromList [Str "a"]}]

header''' works, but it seems to me a hacky solution, and I also don't 
understand *why do I have to exclude '=' in the Pandoc implementation?*
Another problem with header''' is that I can't parse say "=a===" with it.

Of course one may try to use 'try', as follows:

headerWithTry' :: Parser String
headerWithTry' = manyTill anyChar (try (string "==="))

headerWithTry'' :: Parser [Inlines]
headerWithTry'' = manyTill (B.str <$> (many1 anyChar)) (try (string "==="))

headerWithTry''' :: Parser [Inlines]
headerWithTry''' = manyTill (B.str <$> (many1 (noneOf "="))) (try (string 
"==="))

But it does not work:
*Main> simpleParse headerWithTry' "=a==="
Right "=a"
*Main> simpleParse headerWithTry'' "=a==="
Left (line 1, column 6):
unexpected end of input
expecting "==="
*Main> simpleParse headerWithTry''' "=a==="
Left (line 1, column 1):
unexpected "a"
expecting "==="

*Why?*

Thanks,
Jeff

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Re: simple question about using manyTill

[John MacFarlane]

+++ 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?

Re: simple question about using manyTill

[Jeff]

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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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