Wren Thornton wrote:
I had some similar issues recently. The trick is figuring out how to
convince attoparsec to commit to a particular alternative. For example,
consider the grammar: A (B A)* C; where if the B succeeds then we want to
commit to parsing an A (and if it fails then return A's
* Niklas Hambüchen m...@nh2.me [2013-03-16 03:49:29+]
I would agree that what attoparsec does for | of Alternative and mplus
for MonadPlus is correct since e.g. the mplus laws say that a failure
must be identity and therefore the following alternatives must be
considered. I also find it
I think the mistake here is to parse something and then decide if
its it valid. It should be the parser which decides whether its
valid. So rather than:
suffix - A.option ((:) $ A.letter_ascii)
try:
typ - A.choice [ {- list or valid suffix parsers -} ]
return $
On Fri, Mar 15, 2013 at 8:49 PM, Niklas Hambüchen m...@nh2.me wrote:
Is it not possible to add an alternative (no pun intended) to | that
supports the semantics Evan wants?
I assume it's the performance thing. Presumably it would need to pass
an extra flag with to the failure continuation to
@Evan Thanks for that link, I posted a somewhat longer argument in
there.
Personally, I'd love ||.
On Sun 17 Mar 2013 02:02:44 GMT, Evan Laforge wrote:
On Fri, Mar 15, 2013 at 8:49 PM, Niklas Hambüchen m...@nh2.me wrote:
Is it not possible to add an alternative (no pun intended) to | that
I have a couple of problems with attoparsec which I think are related
to its always backtrack nature, but maybe there's some other way to
solve the same problems.
The first is that it's hard to get the right error msg out. For
instance, I have a parser that tries to parse a number with an
On 3/15/13 3:29 PM, Evan Laforge wrote:
However, which error msg shows up depends on the order of the (|)
alternatives, and in general the global structure of the entire
parser, because I think it just backtracks and then picks the last
failing backtrack. Even after carefully rearranging all
Evan Laforge wrote:
The first is that it's hard to get the right error msg out. For
instance, I have a parser that tries to parse a number with an
optional type suffix. It's an error if the suffix is unrecognized:
p_num :: A.Parser Score.TypedVal
p_num = do
num - p_untyped_num
On 16 March 2013 12:54, Erik de Castro Lopo mle...@mega-nerd.com wrote:
Evan Laforge wrote:
The first is that it's hard to get the right error msg out. For
instance, I have a parser that tries to parse a number with an
optional type suffix. It's an error if the suffix is unrecognized:
Is it not possible to add an alternative (no pun intended) to | that
supports the semantics Evan wants?
I would agree that what attoparsec does for | of Alternative and mplus
for MonadPlus is correct since e.g. the mplus laws say that a failure
must be identity and therefore the following
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