Re: [Haskell-cafe] Parsec bug, or...?

On 10/12/09, Derek Elkins wrote:

On Mon, Oct 12, 2009 at 9:28 PM, Uwe Hollerbach wrote:

...

a brain fart?

Hi, cafe, I've been playing a little bit with a small command processor, and I decided it'd be nice to allow the user to not have to enter a complete command, but to recognize a unique prefix of it. So I started with the list of allowed commands, used filter and isPrefixOf, and was happy. But then I increased the complexity a little bit and it got hairier, so I decided to rewrite the parser for this bit in parsec. The function I came up with is

parsePrefixOf n str = string (take n str) >> opts (drop n str) >> return str where opts [] = return () opts (c:cs) = optional (char c >> opts cs)

which I call as

parseFoo = parsePrefixOf 1 "foo"

and it recognizes all of "f", "fo", and "foo" as "foo".

OK so far, this also seems to work fine. But during the course of writing this, I made a stupid mistake at one point, and the result of that seemed odd. Consider the following program. It's stupid because the required prefix of "frito" is only 2 characters, which isn't enough to actually distinguish this from the next one, "fromage". (And if I change that to 2 to 3 characters, everything works fine.) So here's the complete program

module Main where

import Prelude import System import Text.ParserCombinators.Parsec as TPCP

myPrefixOf n str = string (take n str) >> opts (drop n str) >> return str where opts [] = return () opts (c:cs) = optional (char c >> opts cs)

myTest = myPrefixOf 1 "banana" <|> myPrefixOf 1 "chocolate" <|> TPCP.try (myPrefixOf 2 "frito") <|> myPrefixOf 3 "fromage"

myBig = spaces >> myTest >>= (\g -> spaces >> eof >> return g)

parseTry input = case parse myBig "test" input of Left err -> return (show err) Right val -> return ("success: '" ++ val ++ "'")

main = getArgs >>= (\a -> parseTry (a !! 0)) >>= putStrLn

If I compile this, say as program "opry", and run it as shown below, I expect the results I get for all but the last one:

% ./opry b success: 'banana'

% ./opry c success: 'chocolate'

% ./opry fr success: 'frito'

% ./opry fri success: 'frito'

% ./opry fro "test" (line 1, column 3): unexpected "o" expecting "i", white space or end of input

Sooo... why do I get that last one? My expectation was that parsec would try the string "fro" with the parser for "frito", it would fail, having consumed 2 characters, but then the TPCP.try which is wrapped around all of that should restore everything, and then the final parser for "fromage" should succeed. The same reasoning seems to me to apply if I specify 3 characters as the required initial portion for "frito", and if I do that it does succeed as I expect.

So is this a bug in parsec, or a bug in my brain?

Move the try to the last alternative.

No, that doesn't do it... I get the same error (and also the same if I wrap both alternatives in try). Uwe