xml in fptools?

Simon Marlow simonmarhaskell at gmail.com
Fri May 26 06:52:34 EDT 2006

Duncan Coutts wrote:
> On Thu, 2006-05-25 at 16:41 -0700, Ashley Yakeley wrote:
>>Malcolm Wallace wrote:
>>>Oh, I always assume lazy I/O.  It is one of the most useful parts of
>>>Haskell, and I rely on it all the time for both interactivity and
>>>avoidance of space problems.
>>Lazy I/O is problematic and probably a bad idea for libraries:
> Assuming we do have imprecise exceptions what is wrong with lazy IO?

I'm not even certain that lazy I/O doesn't upset referential 
transparency.  It seems hard to construct a concrete counter example 
though.  My intuition is something like this: if evaluating a thunk can 
cause IO to take place, then the act of evaluating that thunk might 
affect the value of another lazy I/O computation, and hence it should be 
possible to get different results by evaluating the thunks in a 
different order.  I'm concerned that in the presence of dependencies 
between lazy I/O computations, the order of evaluation might be visible.

There have been several discussions on this topic, not everyone shares 
my views :-)  eg.


Actually, in a way I really hope I'm wrong.  I would love to use lazy 
I/O, but I believe we need to have a good story for error handling and 
possible evaluation-order issues first.


More information about the Libraries mailing list