This is a very simple demonstration of clojure 1.10's new Datafiable and Navigable protocols, written to aid my own understanding, after reading Sean Corfield's blog post.
This was tested on clojure-1.10.0RC3.
It implements the Datafiable protocol for the Java class
java.nio.file.Path, representing each one as a map containing the
Path's name, size and (if it is a directory) any sub-directories and
files.
The easiest way to try this out is by using
REBL. First, edit deps.edn and set the
path to your REBL jar. Then, start up Clojure.
clj -R:rebl --main cognitect.rebl
At the REPL prompt, type
(use 'nav-demo)
(make-path "/")
In the REBL window, you will see the result of make-path in the left
pane and the datafied version on the right. You can select the
children key and press the rightward-navigation arrow to open up a
list of the subdirectories and files, and again to select one.
Note that the children are returned as a raw sequence of Path
objects, and REBL uses the Datafiable protocol to turn each one in
turn to data. This is a bit ugly. To improve this, I implemented the
Navigable protocol on the children object.
Close REBL and start again:
clj -R:rebl --main cognitect.rebl
At the REPL prompt, type
(use 'nav-demo2)
(make-path "/")
Now the children just appear as file names, but the navigation works the same as before.
The code creates a sequence of Path objects as before, then a
matching sequence of filenames. Navigable is a special protocol in
that it can be implemented by metadata on an object. An implementation
of nav is set on the sequence of filenames, which, when called,
returns the corresponding Path. In other words, it is defined as
follows: (where paths is closed over from the surrounding scope.)
(defn nav [context index value] (get paths index))
When you click on a member of the children sequence, nav gets called
like with context set to the sequence, index is the numerical
index of the item, and value is its filename. Only the index is used
to look up the get the corresponding path.
Hopefully this will be useful.