Hi, I have something similar to the CSV-Parser in RWH, chapter 16. The problem are the possible empty cells, i.e. consecutive '\t'-Characters. -----Original Message----- From: Antoine Latter [mailto:aslatter@gmail.com] Sent: Monday, August 08, 2011 1:44 PM To: Lukasser Stefan (IFAT IMM PSD D TEC / EE) Cc: beginners@haskell.org Subject: Re: [Haskell-beginners] Parse string with optional entries [Parsec] On Mon, Aug 8, 2011 at 6:39 AM, wrote:

Hi all,

I'm trying to parse a string in one of the following forms using Parsec:

"1\t1\t123\t456" -> desired output: [Just 1, Just 1, Just 123, Just 456]

or

"1\t\1\t\t456"  -> desired output: [Just 1, Just 1, Nothing, Just 456]

i.e., the input is a list of numbers, separated by '\t' characters, with the possibility of missing entries. I'm having troubles with the backtracking in case of missing numbers. Can anybody give me a hint?

What do you have so far?

Thanks in advance, Stefan

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Ah, thanks. The next step would then be to replace empty strings with Nothing, and the others with Just Value I forgot to mention that the values are Double. I tried the following: numberP :: Parser Double -- From RWH numberP = do s <- getInput case readSigned readFloat s of [(n, s')] -> n <$ setInput s' _ -> empty <?> "number" valueP :: Parser [Double] valueP = do (many numberP) `sepBy` (char '\t') <?> "value list" It type checks, but raises an exception: Main> parseTest valueP "1\t\t3.3" *** Exception: Text.ParserCombinators.Parsec.Prim.many: combinator 'many' is applied to a parser that accepts an empty string. Is there another way to parse Double values? Thanks a lot so far. -----Original Message----- From: toad3k@gmail.com [mailto:toad3k@gmail.com] On Behalf Of David McBride Sent: Monday, August 08, 2011 3:53 PM To: Lukasser Stefan (IFAT IMM PSD D TEC / EE) Cc: aslatter@gmail.com; beginners@haskell.org Subject: Re: [Haskell-beginners] Parse string with optional entries [Parsec] Read up on the sepBy combinator. parse (sepBy (many digit) (char ',')) "" "123,321,,2321" Right ["123","321","","2321"] On Mon, Aug 8, 2011 at 7:53 AM, wrote:

Hi,

I have something similar to the CSV-Parser in RWH, chapter 16. The problem are the possible empty cells, i.e. consecutive '\t'-Characters.

-----Original Message----- From: Antoine Latter [mailto:aslatter@gmail.com] Sent: Monday, August 08, 2011 1:44 PM To: Lukasser Stefan (IFAT IMM PSD D TEC / EE) Cc: beginners@haskell.org Subject: Re: [Haskell-beginners] Parse string with optional entries [Parsec]

On Mon, Aug 8, 2011 at 6:39 AM,   wrote:

...

Hi all,

I'm trying to parse a string in one of the following forms using Parsec:

"1\t1\t123\t456" -> desired output: [Just 1, Just 1, Just 123, Just 456]

or

"1\t\1\t\t456"  -> desired output: [Just 1, Just 1, Nothing, Just 456]

i.e., the input is a list of numbers, separated by '\t' characters, with the possibility of missing entries. I'm having troubles with the backtracking in case of missing numbers. Can anybody give me a hint?

What do you have so far?

...

Thanks in advance, Stefan

_______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

_______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

On Mon, Aug 8, 2011 at 9:44 AM, wrote:

Ah, thanks. The next step would then be to replace empty strings with Nothing, and the others with Just Value

I forgot to mention that the values are Double. I tried the following:

numberP :: Parser Double  -- From RWH numberP = do    s <- getInput    case readSigned readFloat s of        [(n, s')] -> n <$ setInput s'        _         -> empty   "number"

valueP :: Parser [Double] valueP = do    (many numberP) `sepBy` (char '\t')   "value list"

From looking at your types, I'm not sure how it type checks. Shouldn't this just be:

numbersP `sepBy` (char '\t')

Or, in your case:

optionMaybe numbersP `sepBy` char '\t'

(The second solution should have type Parser [Maybe Double]) Although the error given confuses me - I'm not sure why the 'numbersP' parser should succeed with no input. Antoine

It type checks, but raises an exception:

Main> parseTest valueP "1\t\t3.3" *** Exception: Text.ParserCombinators.Parsec.Prim.many: combinator 'many' is applied to a parser that accepts an empty string.

Is there another way to parse Double values?

Thanks a lot so far.

-----Original Message----- From: toad3k@gmail.com [mailto:toad3k@gmail.com] On Behalf Of David McBride Sent: Monday, August 08, 2011 3:53 PM To: Lukasser Stefan (IFAT IMM PSD D TEC / EE) Cc: aslatter@gmail.com; beginners@haskell.org Subject: Re: [Haskell-beginners] Parse string with optional entries [Parsec]

Read up on the sepBy combinator.

parse (sepBy (many digit) (char ',')) "" "123,321,,2321" Right ["123","321","","2321"]

On Mon, Aug 8, 2011 at 7:53 AM,   wrote:

...

Hi,

I have something similar to the CSV-Parser in RWH, chapter 16. The problem are the possible empty cells, i.e. consecutive '\t'-Characters.

-----Original Message----- From: Antoine Latter [mailto:aslatter@gmail.com] Sent: Monday, August 08, 2011 1:44 PM To: Lukasser Stefan (IFAT IMM PSD D TEC / EE) Cc: beginners@haskell.org Subject: Re: [Haskell-beginners] Parse string with optional entries [Parsec]

On Mon, Aug 8, 2011 at 6:39 AM,   wrote:

...

Hi all,

I'm trying to parse a string in one of the following forms using Parsec:

"1\t1\t123\t456" -> desired output: [Just 1, Just 1, Just 123, Just 456]

or

"1\t\1\t\t456"  -> desired output: [Just 1, Just 1, Nothing, Just 456]

i.e., the input is a list of numbers, separated by '\t' characters, with the possibility of missing entries. I'm having troubles with the backtracking in case of missing numbers. Can anybody give me a hint?

What do you have so far?

...

Thanks in advance, Stefan

_______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

_______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

_______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners