[Haskell-cafe] what is f=f (not) doing ?

[Pasqualino 'Titto' Assini]

On Saturday 22 September 2007 10:58:49 Neil Mitchell wrote:
> Hi
>
> > f = f
> >
> > and then try to evaluate 'f'  in GHCi, as one would expect, the
> > interpreter never returns an answer.
> >
> > The funny thing is that, while it is stuck in an infinite loop, GHCi
> > doesn't seem to use any CPU time at all.
>
> It's called a black hole. The runtime can detect that f directly
> depends on f, so just gives up early. Essentially it marks f as "black
> hole" once it starts evaluating it, and then when it comes back to f,
> it knows its already doing evaluation on f, so just fails. It would be
> useful if there was a page on the wiki about black holes, but I can't
> find one...

Many thanks for the explanation. 

But, if it can detect that is in a black hole, why doesn't it stop the 
computation altogether and return an error to the user?

Best,

     titto

> Thanks
>
> Neil