Hash calculation runs only when necessary, because Clojure's map function is lazy now.
more sample code: (nth (get-info "a.txt") 0) (nth (get-info "b.txt") 0) (nth (get-info "b.txt") 1) result: size a.txt size b.txt quickhash b.txt Output result shows it. When (nth (get-info "a.txt") 0) is evaluated, only get-size function runs. Evaluation of get-quickhash and get-hash is delayed. Eval (nth (get-info "a.txt") 1) cause evaluation of get-quickhash, but not get-hash. 2009/5/28 Timothy Pratley <timothyprat...@gmail.com>: > > Sounds like a job for lazy-map to me! > http://kotka.de/projects/clojure/lazy-map.html > > > On May 28, 11:52 am, Korny Sietsma <ko...@sietsma.com> wrote: >> 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" > > > --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en -~----------~----~----~----~------~----~------~--~---
