Ah, sorry, I get it now - very cute! This is just the sort of thing I need to understand better - hard to break out of years of object-oriented thinking.
- Korny On Thu, May 28, 2009 at 2:34 PM, Mikio Hokari <mikiohok...@gmail.com> wrote: > > 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" >> > >> > > > > -- 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 -~----------~----~----~----~------~----~------~--~---
