On Wed, Feb 18, 2004 at 10:48:39AM +0000, Graham Klyne wrote:
> [To Haskell-cafe...]
>
> At 16:57 17/02/04 -0500, Andrew Pimlott wrote:
> >On Tue, Feb 17, 2004 at 07:48:52PM +0000, Graham Klyne wrote:
> >> [[
> >> notMatching :: Show a => GenParser tok st a -> GenParser tok st ()
> >> notMatching p = do { a <- try p ; unexpected (show a) } <|> return ()
> >> ]]
> >
> > notFollowedBy' :: Show a => GenParser tok st a -> GenParser tok st ()
> > notFollowedBy' p = do res <- do a <- try p; return $ Just a
> > <|>
> > return Nothing
> > case res of Just a -> unexpected (show a)
> > Nothing -> return ()
>
> I don't see why that would work where the above case does not; i.e. when p
> consumes no input.
Because (in my version) when p succeeds, the left side of <|> succeeds;
in the original, the left side of <|> always failed. Instead of
throwing the error immediately after p succeeds, I "package it up" in
Just a and throw it later. So there is no possibility for the right
side of <|> to be evaluated "by accident", as there was in the original.
I think the key is to understand the paragraph in a previous message
about the original implementation being a "dirty trick".
Andrew
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