Malcolm Wallace wrote:
Ross Paterson <[EMAIL PROTECTED]> wrote:
When I've used seq, it's to ensure that a function is strict in that
argument, and therefore has been evaluated before the function is
called. (If the language had unlifted types, I might have used those
instead). Beyond that, I don't care exactly when it was evaluated; it
might be long before, thanks to propagation of strictness information.
When I use `seq`, it is sometimes in a construction like
unsafePerformIO (emit "squawk!) `seq` x
where I am trying to force the impure side-effect to happen, exactly and
immediately before x is evaluated. Whilst this is not good style in a
general sense, I argue that it is perfectly safe inside certain kinds of
library (e.g. for calculating coverage information, or for emitting
tracing information). But if the language itself cannot guarantee this
exact placement of side-effects, then it becomes impossible to write
computation-reflective tools like Hat and hpc for Haskell, in Haskell.
That would surely be a sad state of affairs.
My take on this kind of thing is that if you want a specific operational
behaviour, then you're doing something implementation-specific. We shouldn't
mandate any kind of operational behaviour across Haskell implementations. Yes,
I'm saying you can't do this portably, and it is good that you can't, becauuse
it gives implementations more flexibility.
Cheers,
Simon
_______________________________________________
Glasgow-haskell-users mailing list
Glasgow-haskell-users@haskell.org
http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
