Ross Paterson wrote:
On Mon, Nov 06, 2006 at 06:25:48PM +0000, Malcolm Wallace wrote:
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.
Without admitting the existence of "unsafePerformIO", I submit
unsafePerformIO (emit "squawk! >> return x)
where >> really does imply sequencing.
Does it? What's stopping a Haskell implementation from evaluating x early? As
far as I know, the only time it wouldn't be valid is if x refers to a stream
created by hGetContents, and even then you could evaluate it early as long as
you don't move it past some I/O operation that could "tell the difference"
(although the report doesn't really say this explicitly, it's just the accepted
behaviour of lazy input streams).
Cheers,
Simon
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