Following on from the last post, it actually turns out to be much easier
to do most of the work in Clojure itself — no need for all of that
tiresome messing around with
Symbolss on the Java side
of things! The trick is to define an abstract class in Java to set a few
things up and use as a hook, than implement this in Clojure. I’ll go
through both sides of this, starting with the Java stuff.
The Abstract Class in Java
Basically, I’m using the Java side of things to set up my text pane with a standard stylesheet (I’d like it to use a proportional font, with different colours for input, output, and error text) and to install a key listener to send commands to the Clojure repl whenever the user hits enter or return. The basic class then, is
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createStyledDocument method, which I won’t include here, just
sets up the style context and my colour scheme. The
that is references is an trivial writer subclass that just calls
insertString on the document with the named style. The other class
that I’ll be wanting to use is a runnable so that I can launch the
Clojure REPL on it’s own thread. It’s about as trivial as it gets, it
just calls back into the two abstract methods that I’m going to provide
to provide my Clojure code somewhere to hook onto.
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With this I can provide a start method that installs my key listener and then launches a new thread with an instance of this runnable. The two hook methods that I’m providing are:
abstract void bindVariable(String,Object)to allow me to set up some domain objects on the clojure side of things; and
abstract void doStart(Reader,Writer,Writer)to actually start the REPL, using the provided input, output, and error streams.
The Clojure Implementation
Turns out to be trivial as well, the implementation of
bindVariable just interns the object passed in into the user
namespace, it’s a one liner in Clojure.
doStart method isn’t much more involved either, it just sets up
the bindings and then launches the REPL.
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Notice that here I have added type annotations so that the correct
method gets implemented, without these the Clojure code compiled but
then I got abstract method errors at runtime. Check out the docstring
repl function as well, there are a few useful options (for
example in my actual code I have an
:init function to switch to the
user namespace, and a custom prompt). For completeness, here’s the rest
of the Clojure file with the code required to inherit from the Java base
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This approach has the advantage that any additional configuration can happen in the clojure code. It would also be easy, for example, to have an additional script that was always run at start up, to allow the user to customize the console further (similar to the .emacs file in Emacs). You could also move most of the work that I’m doing in Java into the Clojure code. I haven’t done this as I may want to support multiple languages in my console and it’s nice to have a common stylesheet and keybindings (e.g. for history) across languages. Your mileage may vary.