2013/3/13 Mark Engelberg <mark.engelb...@gmail.com>
> Still, it would be nice to understand whether it's possible to achieve the
> same effect as Clojure core's settable vars.
>
It is, because clojure doesn't do anything magic here (apart from the
effects its set!-able vars have ;-).
The catch is: The set of commonly set!-able vars is tied to clojure's
release version. And as we all know, this context is controlled by
clojure/core, in the hope of ruling out bad majority decisions.
Proof by command line:
> java -cp clojure-1.5.0.jar clojure.main -h
... snip ...
operation:
- Establishes thread-local bindings for commonly set!-able vars
... snip ...
So presumably, you would have to get the var you want to have "globally"
set!-able into clojure.main, or lobby for a general purpose hook to
globally add dynamic bindings to a thread (a bad idea in my gut intuition).
*user=>* (.start (Thread. #(set! *unchecked-math* true)))
nil
Exception in thread "Thread-3" java.lang.IllegalStateException: Can't
change/establish root binding of: *unchecked-math* with set
at clojure.lang.Var.set(Var.java:233)
;; so no, clojures set!-able vars aren't magically carried over to new
threads either
Opinions
IMO set!-able vars only make sense, if you control the context in which the
vars are set!-able. Dynamic bindings are plenty volatile as they are,
especially in combination with lazy sequences (just happened to me again
today). The ability to introduce *new* bindings *across* or *up* the
dynamic scope certainly wouldn't help there.
I think the easiest way to offer your users to control part of their
context (to "globally" introduce set!-able vars and other stuff) are:
leiningen plugins.
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