Is it possible to override the behavior of function application in Clojure?
For instance I'd like to make a namespace where:
(+ 2 2)
Evaluates to the constant 5 via a macro like:
(defmacro app [& args] 5)
1 Answer
+ is just a function so you can, for your namespace, exclude referring to clojure.core/+ and then define your own addition function in that namespace.
Clojure does not offer a general hook for the concept of function application in this namespace. What the development tools do is read the namespace for all the symbols that contain functions, and then hook them with wrapping function that does the tracing. This only affects functions that exist in the namespace at the time this happens, so it's less general than what you are asking for.
see clojure.tools.trace and Robert Hooke for more examples of this.
6 Comments
Ben Greenman
That's great, but can I override application?
Arthur Ulfeldt
Do you mean something other than the effect of calling a function when you say application here? Perhaps an example with the desired output would help me understand
Arthur Ulfeldt
Are you asking if you can alter the behavior of all function calls everywhere?
Ben Greenman
Yes. I am asking if I can alter the behavior of all function calls in a namespace. Example use: executing a print statement every time a function is called when the code runs.
Arthur Ulfeldt
I added this to my answer
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appand the form(+ 2 2). Please clarify exactly what it is you're trying to do.appto change the behavior of every function application in the namespace. Including(+ 2 2)and also(+ 1 2 3)and(f x)for any functionf.