On 15 April 2004 05:20, Stefan Ljungstrand wrote:
> The code is a small modification of mine to the attribute-grammar-like
> (multi-pass lazy) example with recursive IO at
> http://www.cse.ogi.edu/PacSoft/projects/rmb/repMin.html
>
> The original code replaces every (leaf) elements in a tree by the
> minimum
> of them all, returning the new tree, while printing out the elements,
> in traversion order.
>
> My modification tries to also print out minimum (at each non-leaf
> call)
> the global minimum (which is lazily passed down as a value from the
> 'future'). Probably this should generate a "Fail: <<loop>>" at first
> attempt to prematurely force the global minimum (or maybe earlier).
>
> The problem is, it actually duplicates some of the other IO effects a
> few times, before grinding to a "Fail: <<loop>>" stop !
What is happening here is that your program goes into an infinite loop
(as expected), but GHC doesn't notice the loop for a while. This is
because GHC does something called "lazy blackholing". When it does
finally notice the loop, you get a NonTermination exception.
Probably the correct thing to do here is for GHC to guarantee to give
the black hole exception immediately. I believe this is what the fixIO
semantics proposed by Erkok et. al. specifies (Levent or John - can you
comment?).
We can define fixIO like this, which does what you want:
fixIO :: (a -> IO a) -> IO a
fixIO k = do
ref <- newIORef (throw NonTermination)
ans <- unsafeInterleaveIO (readIORef ref)
result <- k ans
writeIORef ref result
return result
Cheers,
Simon
_______________________________________________
Glasgow-haskell-bugs mailing list
[EMAIL PROTECTED]
http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
