[Haskell] Parsec question: attempted 'notMatching' combinator

[Graham Klyne]

Thanks!  That got me going, though not with quite what you suggested.

I ended up with this:
[[
notMatching :: Show a => GenParser tok st a -> GenParser tok st ()
notMatching p = do { a <- try p ; unexpected (show a) } <|> return ()
]]
which does the required job for me.

Using your version caused the notMatching parser to be equivalent to:
   return ()
presumably, because the failure was protected by the try combinator?

#g
--

At 14:29 17/02/04 -0500, Andrew Pimlott wrote:
>On Tue, Feb 17, 2004 at 06:42:39PM +0000, Graham Klyne wrote:
> > I've attempted to define a Parsec combinator thus:
> >
> > [[
> > notMatching :: Show a => GenParser tok st a -> GenParser tok st ()
> > notMatching p = try ( do { a <- p ; unexpected (show a) } <|> return () )
> > ]]
>
>If p fails but consumes some input, return () won't be tried, thus the
>whole notMatching will have failed.  The try means that it will not have
>consumed any input, but that's no consolation, because notMatching
>should have succeeded!  How about:
>
>     notMatching p = try ( do { a <- p ; unexpected (show a) } ) <|> return ()
>
>In fact, I think notFollowedBy can be considered to be buggy in the same
>way.
>
>     notFollowedBy p     = try (do{ c <- p; unexpected (show [c]) }
>                                <|> return ()
>                               )
>
>It doesn't usually bite, because notFollowedBy takes a parser that
>returns a token, and such a parser normally only looks at one token.
>But it could look at more:
>
>     aNoBC = do  char 'a'
>                 notFollowedBy $ do  char 'b'
>                                     char 'c'
>
>Intutively, (aNoBC >> char 'b') should match "abe", but
>
>     *Main> parseTest (aNoBC >> char 'b') "abe"
>     parse error at (line 1, column 2):
>     unexpected "e"
>     expecting "c"
>
>If you instead put the try around the do, it works as expected.  So I
>conclude that the try simply got put in the wrong place by mistake.
>
>Andrew

------------
Graham Klyne
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