[plt-scheme] why should black holes be exceptions, rather than divergence?

Errors seem to always be preferable to divergence to me.

Robby

On Mon, May 31, 2010 at 8:09 AM, Keiko Nakata
<keiko at kurims.kyoto-u.ac.jp> wrote:
> Hello,
>
> I have been thinking about Scheme's implementation of "black holes",
> or provable divergence, as errors. That is,
>
>> (letrec [(x (delay (force x)))] (force x))
>
> raises an error rather than diverges.
>
> Can someone refer me to or give me historical background or motivations
> of this particular design choice?
>
> Haskell has recursive monadic bindings, (aka value recursion?), which
> implement black holes as divergence (to some extent). That is,
>
> ghci> import Control.Monad.Fix
>
> ghci> (mfix (\x -> do print "hi" >>  return (x+1)))
>
> prints "hi" once and diverges.
> (See more examples at the bottom.)
>
> Scala's lazy val has yet another behavior.
>
> scala> lazy val x: int = { println("hi"); x}
>
> diverges (until the stack overflows) while continues printing "hi" again and
> again...
> (For reference
>
> scala> lazy val x: int = { println("hi"); 1}
> scala> x
>
> prints "hi" just once.)
>
> I can guess what implementation respectively gives each of the above
> behaviors. I just have to tweak the default value put in the
> initialization-thunk.  But I do not know what considerations led each
> language to these different implementations.
>
> Any idea?
>
> Keiko
>
>> (mfix (\x -> do print "hi" >>  return (x+1)))
>
> prints "hi" once and diverge.
>
>> (mfix (\x -> do print "hi" >>  return x))
>
> prints "hi" once and terminates.
>
>> (mfix (\x -> do print "hi" >>  x >> return x))
>
> prints "hi" once and raises an exception.
>
>
>
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