[Haskell-cafe] Parsec (Zero or One of)
Hi,
I'm having some difficulty using the Parsec library, perhaps you could
help. I've reduced my problem as shown below. I would like the
'only_prod' parser to require the reserved string only, _optionally_
followed by an identifier. As part of 'mytest', this should then be
followed by the reserved string end.
mytest = do { only_prod; end }
only_prod = do { reserved only; try identifier }
end = reserved end
i.e. I'd like both
parseTest mytest only end
and
parseTest mytest only green end
to parse successfully. As it stands, only the second is successful. The
first fails with:
parse error at (line 1, column 9):
unexpected end of input
expecting letter or digit or end
Does anyone have an idea how I could repair the situation?
Thanks in advance,
Paul
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Re: [Haskell-cafe] Parsec (Zero or One of)
Paul Keir wrote: I’m having some difficulty using the Parsec library, perhaps you could help. I’ve reduced my problem as shown below. I would like the ‘only_prod’ parser to require the reserved string “only”, _optionally_ followed by an identifier. As part of ‘mytest’, this should then be followed by the reserved string “end”. Look into option, optional, and optionMaybe. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Parsec (Zero or One of)
On Tue, 2008-03-25 at 11:47 -0400, Albert Y. C. Lai wrote: Paul Keir wrote: I’m having some difficulty using the Parsec library, perhaps you could help. I’ve reduced my problem as shown below. I would like the ‘only_prod’ parser to require the reserved string “only”, _optionally_ followed by an identifier. As part of ‘mytest’, this should then be followed by the reserved string “end”. Look into option, optional, and optionMaybe. And in particular, try doesn't do what you (Paul) think it does. try however is a rather important combinator of Parsec so I would behoove you to look up it's documentation and the explanations of it in the Parsec letter. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Parsec (Zero or One of)
Thanks. I can't find optionMaybe in my version 2.1 of Parsec, but in any case,
defining my only_prod as
only_prod = do { reserved only; option [] identifier }
or
only_prod = do { reserved only; identifier | return [] }
gives the same error responses as before. I will anyway look closer at option.
You're right that I don't understand try, but it's not for lack of trying. My
examples' use of try though was just a stab at a readable failure. Maybe I
should refactor my example.
Paul
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Re: [Haskell-cafe] Parsec (Zero or One of)
On Mar 25, 2008, at 12:43 , Paul Keir wrote:
Thanks. I can't find optionMaybe in my version 2.1 of Parsec, but
in any case, defining my only_prod as
only_prod = do { reserved only; option [] identifier }
or
only_prod = do { reserved only; identifier | return [] }
gives the same error responses as before. I will anyway look closer
at option.
The other problem here is that just using a given string in
reserved doesn't prevent it from being parsed elsewhere by
identifier. (Note the character offset of the error was 9, i.e.
just past only end, and it was looking for end or more identifier
characters.)
Are you using the higher level parser facilities from
Text.ParserCombinators.Parsec.Token, or rolling your own? If the
latter, you will need to modify identifier to not accept keywords.
--
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED]
system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED]
electrical and computer engineering, carnegie mellon universityKF8NH
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RE: [Haskell-cafe] Parsec (Zero or One of)
Thankyou. Yes, I'd also noticed that only end could result in the end part
being taken as an identifier. The language I'm parsing actually doesn't have
reserved words though; so end and only are both possible valid identifiers.
I should then probably replace my use of say, reserved only, with string
only; whiteSpace; for clarity. Still stuck though...
P
-Original Message-
From: Brandon S. Allbery KF8NH [mailto:[EMAIL PROTECTED]
Sent: Tue 25/03/2008 19:58
To: Paul Keir; haskell-cafe@haskell.org Cafe
Subject: Re: [Haskell-cafe] Parsec (Zero or One of)
On Mar 25, 2008, at 12:43 , Paul Keir wrote:
Thanks. I can't find optionMaybe in my version 2.1 of Parsec, but
in any case, defining my only_prod as
only_prod = do { reserved only; option [] identifier }
or
only_prod = do { reserved only; identifier | return [] }
gives the same error responses as before. I will anyway look closer
at option.
The other problem here is that just using a given string in
reserved doesn't prevent it from being parsed elsewhere by
identifier. (Note the character offset of the error was 9, i.e.
just past only end, and it was looking for end or more identifier
characters.)
Are you using the higher level parser facilities from
Text.ParserCombinators.Parsec.Token, or rolling your own? If the
latter, you will need to modify identifier to not accept keywords.
--
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED]
system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED]
electrical and computer engineering, carnegie mellon universityKF8NH
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Re: [Haskell-cafe] Parsec (Zero or One of)
On Mar 25, 2008, at 16:26 , Paul Keir wrote: Thankyou. Yes, I'd also noticed that only end could result in the end part being taken as an identifier. The language I'm parsing actually doesn't have reserved words though; so end and only are both possible valid identifiers. I should then probably replace my use of say, reserved only, with string only; whiteSpace; for clarity. Still stuck though... But now you have an ambiguity in your language, which is exactly why the parse is failing: only end could be waiting for end, or for end of file / whatever tokens might follow this clause. In the worst case, the latter might lead to a situation where an unambiguous parse is impossible. You might want to provide a better description of the full language --- and think about how it would need to be implemented to avoid ambiguity. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED] system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED] electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Parsec (Zero or One of)
Many thanks. -Original Message- From: Brandon S. Allbery KF8NH [mailto:[EMAIL PROTECTED] Sent: Tue 25/03/2008 20:29 To: Paul Keir; haskell-cafe@haskell.org Cafe Subject: Re: [Haskell-cafe] Parsec (Zero or One of) On Mar 25, 2008, at 16:26 , Paul Keir wrote: Thankyou. Yes, I'd also noticed that only end could result in the end part being taken as an identifier. The language I'm parsing actually doesn't have reserved words though; so end and only are both possible valid identifiers. I should then probably replace my use of say, reserved only, with string only; whiteSpace; for clarity. Still stuck though... But now you have an ambiguity in your language, which is exactly why the parse is failing: only end could be waiting for end, or for end of file / whatever tokens might follow this clause. In the worst case, the latter might lead to a situation where an unambiguous parse is impossible. You might want to provide a better description of the full language --- and think about how it would need to be implemented to avoid ambiguity. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED] system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED] electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
