Lazy evaluation & performance
 Benoit de Boursetty
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Date:  20000221 (17:13) 
From:  Benoit de Boursetty <debourse@e...> 
Subject:  Lazy evaluation & performance 
Hello, Has anybody done benchmarks to eval the cost of lazy computation encapsulation, in terms of time, memory, garbage collection? I have no idea of how this is implemented... Here's my personal case : There is a function f which I want to compute for several arguments x_1,...x_n. let f x = [beginning] let intermediate_value = ... in [end] only that I want to compute it thoroughly just for the x_i that has the highest intermediate_value among x_i's. This intermediate value is used anyway for the [end] part. Naive design (design a): let f x = [beginning] let intermediate_value = ... in (intermediate_value, lazy [end]) the lazy computation of the [end] being forced only for the x_i that has the highest intermediate_value Another possible design (less elegant) (design b): let intermediate_value x = [beginning] let intermediate_value = ... in intermediate_value let f x = [beginning] [end] I compute the intermediate value and then recompute all over again for the x I want to compute. Comparison of time costs: if B is the cost for [beginning] E is the cost for [end] L is the cost for encapsulating the lazy computation of [end] then design a costs n*(B+L) + E design b costs (n+1)*B + E (very roughly I suppose) Clearly, deciding which design to adopt is a tradeoff depending on n, B, L. I suppose L also depends on the number of results from [beginning] that the computer will need to "remember" for [end]? Also, encapsulating lazy computations means more memory allocation, means more garbage collecting, doesn't it? In my case the efficiency bottleneck is E not B, and n is about 10 (i.e. high) so I'm not expecting a wonderful overall time gain. I'm just wondering if it's costly to implement it in the way that corresponds best to reality (design a). "B" is only a dozen flops. Could anybody give me a hint about the order of magnitude of L? Thanks very much in advance for your answers. Benoit de Boursetty.