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 -~----------~----~----~----~------~----~------~--~---
