Question

Is there any way to specify the error message when using set-
validator! with a dynamic var?

In a situation like this:

(binding [*my-var* -1]
  (foo))

I’d like to be able to throw a custom exception when they try to bind
to a negative number.

Using set-validator! like this, the Exception message is ignored:

(set-validator! #'*d* (fn [x] (if (< x 0) (throw (Exception. "*my-var*
can only be bound to a non-negative number")) true)))

Instead the message you see at the REPL is:
java.lang.IllegalStateException: Invalid reference state
(NO_SOURCE_FILE:0)

Is there a way to specify the error message on validation-error of a
dynamic var?

Answer 1

If you use a RuntimeException instead of an Exception the error mesage will be the message inside of your RuntimeExecption.

(set-validator! #'*d* (fn [x] (if (< x 0) (throw (RuntimeException. "*my-var*
can only be bound to a non-negative number")) true)))