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.
David
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