Wow, it's like I'm reading Let Over Lambda all over again :) On Mar 15, 9:58 am, Armando Blancas <armando_blan...@yahoo.com> wrote: > Another choice is to construct shapes as closures with auto-dispatch. > So a circle could be made thus, with no data structure per se: > > (defn make-circle [x y r] > (fn [method] > (case method > :draw (fn [color] ...) > :rotate (fn [degrees] ...) > :r (fn [] r) > :x (fn [] x) > :y (fn [] y)))) > > Then the API would just delegate to each object: > > (defn draw [shape color] > ((shape :draw) color)) > > (defn rotare [shape degrees] > ((shape :rotate) degrees)) > > And is used like this: > > (def circle (make-circle 5 5 10)) > (draw circle) > > On Mar 15, 9:26 am, Alan <a...@malloys.org> wrote: > > > > > > > > > On Mar 14, 7:54 pm, stu <stuart.hungerf...@gmail.com> wrote: > > > > Hi, > > > > I'd like to create a simple library of drawable shapes: lines, circles > > > and rectangles. I've placed each type of shape in its own namespace > > > with functions that operate on that shape kind: > > > > (ns myshapes.line) > > > > (defn line ... creates new line ...) > > > > (defn draw ... draws a line ...) > > > > To keep things simple a line is just a vector of two points and > > > circles and rectangles are just structs. > > > I'd also like to have a sequence of shapes called a picture: > > > > (ns myshapes.picture) > > > > (defn picture ... returns picture sequence from args ...) > > > > (defn draw ... draws whole picture) > > > > What's the idiomatic way of handling a situation like this in > > > Clojure? Do I need to use richer data structures than vectors and > > > structs for the shapes so they carry some kind of type information? I > > > can see how a draw multi-method would work if the individual shapes > > > could be distinguished, or am I going about this the wrong way? > > > I think what you need is *less* rich data types, as I outline in my > > answer (sorry, didn't notice this part of the question or I'd have > > combined the two). Instead of special structs for each type, just have > > every shape carry information in similar ways; then you can write a > > draw-shape function that works with any old shape, not needing to know > > much about them. If you want to be more explicit you can define all > > shapes as a struct of {:points [p1 p2...], :whatever more-data, :and- > > also even-more}, but it will help your dispatch immensely if there's a > > single, coherent way to get a shape's points regardless of its types. > > > Then if you ever have to put special logic that depends on something > > other than the points in the shape (eg circles have a radius), you can > > turn that draw-shape function into a multimethod and have the :default > > dispatch value do all the "boring" drawing.- Hide quoted text - > > > - Show quoted text -
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