How to memoize functions?

[Chris Reedy]

Jerome Alet wrote:
> Le Thu, 26 Jun 2003 16:29:07 -0400, Chris Reedy a écrit :
> 
> 
>>The obvious way to memoize a function would be to keep a dictionary with
>>keys being tuples (or maybe dictio naries) of previous argument lists
>>and values being the results of the previous computations.
>>
>>Unfortunately, this will really mess up garbage collection, since
>>objects that are arguments could never be collected.
> 
> 
> first I must say that my answer is probably completely stupid.
> 
> however why would you want to use the original arguments tuple as a key ?

Actually, I chose that just because it was the most obvious. The next 
choice was a tuple of the ids of the arguments ...

> depending on the computational intensity of your original function, why
> not compute some sort of hash (like an hex md5sum) on all the arguments
> (converted to strings and concatenated) ?

This is certainly a possibility. However ...

> this way your original arguments would still be garbage collectable (well,
> at least I suppose), since only the hash would be used for the key.

That's true. Unfortunately, that misses the other half of the problem 
(which, admittedly, I didn't mention) which is that I would also like to 
be able to collect the results of the function, which could be complex 
data structures, as well as the arguments (which could be other 
instances of the same complex structures).

   Chris



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