[Haskell-cafe] DFAs and self-referential data

[Maxime Henrion]

On Sun, 2010-12-26 at 13:58 +0200, Roman Cheplyaka wrote:
> * Maxime Henrion <mhenrion at gmail.com> [2010-12-26 12:01:31+0100]
> > Anyone knows what I'm doing wrong here?  I suspect my attempt at having
> > self-referential data is somehow buggy; do I need to treat transitions
> > to the same state differently?
> 
> The problem is that when you call 'self', you record *that* state of
> your DFA in the map. When DFA gets updated further, the recorded
> self-reference is not updated appropriately.
> 
> In your case a workaround is to call 'self' after all the other updates,
> i.e.
> 
>     test :: String -> Bool
>     test = accept s1
>       where s1 = self '1' . path '0' s2 $ empty True
>             s2 = self '1' . path '0' s1 $ empty False
> 
> But I don't see why you need 'self' at all -- you can just use path as
> with any other type of transition:
> 
>     test :: String -> Bool
>     test = accept s1
>       where s1 = path '0' s2 . path '1' s1 $ empty True
>             s2 = path '0' s1 . path '1' s2 $ empty False

Indeed this just works, thanks!  The reason I was using a 'self'
function was that I initially thought it would be more convenient; I now
see it doesn't, especially considering it doesn't even work.  However
I'm a bit confused as to why things just work without having to reorder
the calls when using the 'path' function - my brain seems to have
difficulties following the code path here :-).

Cheers,
Maxime