FixIO/ Tackling Awkward Squad

Levent Erkok [email protected]
Fri, 16 Feb 2001 10:01:06 -0800


I recently wrote:

> So, it looks like:
> 
>   fixIO m = let x = unsafePerformIO (m x) in return x
> 
> will do a better job.

But of course, I forgot to make my point before sending the message!

The non-strict version is not good either, because it won't do the
effects!

Consider the definition:

  fixIO m = let x = unsafePerformIO (m x) in return x   

and the expression:

   do { fixIO (\x -> do {putStr "hello"; return (x+1)}); return 2 }

We want this to first print hello and then return 2. The non-strict
version
will return the 2, but will not print the "hello". And the strict
version will
print the "hello" but won't return the 2. We want both. Hence neither
definition
is sufficient.


-Levent.