[Haskell-cafe] Thompson's Exercise 9.13

[Daniel Fischer]

Am Sonntag, 10. April 2005 17:33 schrieben Sie:
> On 2005/04/10, at 18:18, Daniel Fischer wrote:
> >> But this returns [] and doesn't work. I can't get "left" to know that
> >> it is working with the rightmost element. It just throws away every
> >> element when its right hand side is empty.
> >>
> >> I found a solution that works for Strings, but I am looking for a more
> >> general solution. This exercise may yet again be one of those that is
> >> difficult to solve with my current knowledge of Haskell, but I'm
> >> asking
> >> anyway.
> >
> > I cannot imagine how you could do it for Strings, but not for general
> > lists.
> > Mind sending me the code?
>
> The code is here:
> http://wiki.pragprog.com/cgi-bin/wiki.cgi/ThompsonNineAnswersJD Look
> for 9.13 towards the bottom of the page.
>
> It uses chr 0 as an end of string marker, and so this solution only
> works for Strings.

Unfortunately, Strings may contain '\0':

*Init> finit "this doesn't\0quite\0work\0!"
"this doesn't\NULquite\NULwor"

so it doesn't work in all cases. The idea is nice, though:

import Data.Maybe

finit  = catMaybes . foldr f [Nothing] . map Just
  where
  f x ys = case ys of
                    [Nothing] -> []
                    _             -> x:ys

will do fine.
>
> --
> Kaoru Hosokawa
> khosokawa at gmail.com

Cheers,
Daniel