Hi all,
I have some ruby code that I'm thinking of porting to clojure, but I'm
not sure how to translate this idiom to a functional world:
I have objects that are externally immutable, but have internal
mutable state they use for optimisation, specifically in this case to
defer un-needed calculations.
Basically, I have a FileInfo class that wraps a data file, used to
compare lots of files on my system.
It has an "exact_match" method similar to:
def exact_match(other)
return false if size != other.size
return false if quickhash() != other.quickhash()
return hash() != other.hash()
end
quickhash and hash store their results in instance variables so they
only need to do the expensive calculations once - and quite often they
never need to get calculated at all; I'm looking for duplicate files,
but many files have no duplicate, so probably never need to have their
contents hashed.
How would I do this in a functional way? My first effort would be
something like
(defn hash [filename] (memoize (... hash function ...)))
but I have a couple of problems with this:
- it doesn't seem to store the hash value with the rest of the file
information, which feels a bit ugly
- I assume it means storing the full filename three times, once in
the original file info structure, once in the memoized hash function,
and once in the memoized quickhash function. My program struggles to
get enough RAM to track as many files as I'd like already - storing
the filename multiple times would blow out memory quite badly.
I guess I could define a unique key for each filename, and define hash
as a function on that key, but then hash would need to be able to
access the list of filenames somehow. It's starting to get beyond me
- I'm hoping there's a simpler option!
Any suggestions? I'd hope this is not an uncommon idiom.
- Korny
--
Kornelis Sietsma korny at my surname dot com
"Every jumbled pile of person has a thinking part
that wonders what the part that isn't thinking
isn't thinking of"
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