After some private exchange of info between Uwe and me it became clear
that it may not have been immediately clear that the error messages in
my original posting where actually part of a more involved process.
By removing the optional part in the pCommand (i.e. making the part
starting with `opt` invisible, we can show the full power of the built-
in error correction.
import Text.ParserCombinators.UU.Parsing
pCommand [] = pure []
pCommand (x:xs) = ((:) <$> pSym x <*> pCommand xs) -- `opt` (x:xs)
pCommands = amb . foldr (<|>) pFail . map pCommand $ ["banana",
"chocolate", "frito", "fromage"]
t :: String -> ([String], [Error Char Char Int])
t input = parse ( (,) <$> pCommands <*> pEnd) (listToStr input)
Now one also gets error messages like:
*Main> t "frxi"
(["frito"],[
Deleted 'x' at position 2 expecting one of ['i', 'o'],
Inserted 't' at position 4 expecting 't',
Inserted 'o' at position 4 expecting 'o'])
for free.
However, since the decision what element to take is based on a limited
look-ahead, one also gets:
*Main> t "xfxrxix"
(["fromage"],[
Deleted 'x' at position 0 expecting one of ['b', 'c', 'f', 'f'],
Deleted 'x' at position 2 expecting 'r',
Deleted 'x' at position 4 expecting 'o',
Deleted 'i' at position 5 expecting 'o',
Deleted 'x' at position 6 expecting 'o',
Inserted 'o' at position 7 expecting 'o',
Inserted 'm' at position 7 expecting 'm',
Inserted 'a' at position 7 expecting 'a',
Inserted 'g' at position 7 expecting 'g',
Inserted 'e' at position 7 expecting 'e'])
*Main>
which is something not completely expected; the current look-ahead is
however three symbols ahead, and once a decision is taken this is not
reconsidered (for cost reasons). This is currently a consequence of
the rather simplistic inner organisation of the intermediate library.
In the next version we hope to have gotten rid of this artefact.
Best,
Doaitse
Yes, for my particular problem the complexity is very limited. I
wouldn't even have used parsec for this, in spite of the comment I had
made earlier about this, if I were not already using it in a different
part of the project to parse individual records ("buy security <foo>
for this price on this date", etc), so it was natural to add a bit
more parsec code to also deal with the commands saying what I want to
see from the data. It's all still pretty trivial, but starting already
to be useful to me... it's really quite gratifying what a small amount
of haskell code suffices to make a useful and flexible program.
best regards,
Uwe
On 10/15/09, S. Doaitse Swierstra <[email protected]> wrote:
On 15 okt 2009, at 16:58, Uwe Hollerbach wrote:
Hi, all, thanks for the further inputs, all good stuff to think
about... although it's going to be a little while before I can
appreciate the inner beauty of Doaitse's version! :-)
The nice thing is that you do not have to understand the inner
workings ;-} I basically builds a greedy parser for each word to be
recognised which can stop and assume the rest is there if it can no
longer proceed (the `opt` is greedy in its left alternative) . Hence
it recognises the longest possible prefix.
Since my parsers pursue all alternatives in parallel you
automatically
get what you want, without having to indicate prefix lengths, calls
to
try, etc.
The "amb" combinator has type
amb :: Parser a -> Parser [a]
and collects the result from all alternatives its argument parser is
constructed from; you might say it convert an ambiguous parser to a
parser with a list as result, hence preventing the rest of the input
being parsed over and over again. I am currently working on bringing
back more abstract interpretation in the implementation (i.e. what we
have had for almost 10 years in the uulib library), but I do not
expect you to see a lot of that from the outside.
If you want to work with left-recursive parsers (which does not seem
to be the case), you may revert to more complicated solutions such as
found in the "christmastree" (Changing Haskell's Read Implementation
Such That by Manipulationg Abstract Syntax Trees Read Evaluates
Efficiently) package if you need to generate parsers online, or to
happy-based solutions in case your grammar is fixed.
If you have any questions do not hesitate to ask,
Doaitse
I had considered
the approach of doing a post-parsec verification, but decided I
wanted
to keep it all inside the parser, hence the desire to match prefixes
there (and lack of desire to write 'string "p" <|> string "pr" <|>
string "pre" ...'.
By way of background, the actual stuff I'm wanting to match is not
food names, but some commands for a small ledger program I'm working
on. I needed something like that and was tired of losing data to
quicken every so often. I realize of course that there are other
excellent ledger-type programs out there, but hey, I also needed
another hacking project. I'll put this onto hackage in a while, once
it does most of the basics of what I need. No doubt the main
differentiator between mine and those other excellent ledger
programs
out there will be that mine has fewer features and more bugs...
thanks again, all!
Uwe
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