seq vs. pseq

Simon Marlow simonmarhaskell at
Tue Nov 7 04:58:03 EST 2006

Malcolm Wallace wrote:
> Ross Paterson <ross at> 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.


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