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 wrote:

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On 15 okt 2009, at 16:58, Uwe Hollerbach wrote:

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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

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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