Hello David, 2011/8/5 David Nolen <dnolen.li...@gmail.com>
> On Wed, Aug 3, 2011 at 5:26 AM, Rickard Lindberg <ricl...@gmail.com>wrote: > >> Hi, >> >> I am interested in Clojures approach to managing state and its use of >> immutable >> values. I believe immutable values will make the life of programmers >> easier and >> I'm trying to figure out how I can simplify my OO code by using more >> immutable >> values. >> >> In particular, I am wondering how I can model notes that can belong to a >> category. >> >> So a note is a piece of text and a reference to a category. A category is >> just >> a name. However, both notes and categories are entities (the identity is >> *not* >> defined by their value). So it is perfectly ok to have two notes with the >> same >> text and same category, yet they are different notes. >> >> The data structure holding these things together (lets call it NoteDb) has >> a >> list of categories and a list of notes. And I want to be able to say >> things >> like change the text of a note to this. And when I change the name of a >> category, all notes belonging to that category know that the category name >> has >> changed. I also want to be able to get all notes belonging to a category. >> >> Should I model this as a list of refs? Is there another way to think about >> this >> problem that I don't see because I have mainly done work in OO languages. >> >> -- >> Rickard Lindberg >> > > I'm biased but I think core.logic is a pretty good way to do this kind of > thing. > > (ns clojure.core.logic.db > (:refer-clojure :exclude [inc reify ==]) > (:use [clojure.core.logic minikanren prelude])) > > (defrel category ^:index id ^:index name) > > (facts category > [[1 'idea] > [2 'todo]]) > > (defrel notes ^:index id ^:index category ^:index text) > > (facts notes > [[1 1 "My cool widget"] > [2 1 "My cool Clojure project"] > [3 2 "Walk the dog"]]) > > (defn notes-for-category [name out] > (exist [cid nid t] > (category cid name) > (notes nid cid t) > (== [name t] out))) > > (comment > (run* [q] (notes-for-category 'idea q)) > ;; ([idea "My cool widget"] [idea "My cool Clojure project"]) > > ;; rename a category > (fact category 1 'thought) > > (run* [q] (notes-for-category 'thought q)) > ;; ([thought "My cool widget"] [thought "My cool Clojure project"]) > ) > > In order to flesh out this use case I'd have to hear a lot more feedback > about what people would like to see. > What kind of "strict control over state mutation" stories can we expect when using core.logic ? I mean, in the above example, I see no explicit use of refs, and I see somewhat "unmanaged" state change with the introduction of the new category name. This does not look like idiomatic clojure to me (from 10 000 feets, I havent' studied core.logic yet) Cheers, -- Laurent -- 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 Note that posts from new members are moderated - please be patient with your first post. 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
