[Haskell-cafe] Strict and non-strict vs eager and lazy
Malcolm.Wallace at cs.york.ac.uk
Mon Jul 18 11:41:58 EDT 2005
"Bayley, Alistair" <Alistair_Bayley at ldn.invesco.com> writes:
> > > Lazy evaluation is an implementation technique which
> > > satisfies non-strict semantics, but it is not the only
> > > technique which does this.
> where can I find information about non-lazy implementation of non-strict
Remember that laziness = non-strictness + sharing. Thus, one valid
but non-lazy implementation strategy for Haskell would be to avoid
let a = something in a+a
you could evaluate 'a' twice. That might seem silly, because it
wastes work. But there are potential benefits:
* A multi-processor parallel implementation would not need to put
locks around computation that is already underway. Locks can be
expensive and tricky to get right - just doing the computation
twice might be cheaper.
* If the shared value is large and persists for a long time but
is only used infrequently, it might be considered a space leak.
If the heap is nearly full, it could be cheaper to throw the
value away immediately and recalculate it each time it is needed.
A reduction strategy that /always/ avoided sharing would be unlikely
to have great performance, but one that was selective about sharing
(based on either fancy analysis, or empirical profiling) could possibly
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