Print multi method dispatch values in the exception messages.

Question

The error messages of defmulti are at the moment not as helpful as they could be under certain circumstances. Calling this multi method with a lazy seq as it's dispatch argument raises the following exception:

`
(defmulti test-multi identity)
(defmethod test-multi 1 [arg] true)

(test-multi (map identity [:x]))
;=> java.lang.IllegalArgumentException: No method in multimethod 'test-multi' for dispatch value: clojure.lang.LazySeq@3c6f1187
`

Sometimes it would be useful to actually see which values are in the lazy seq being dispatched on. A better error message could look like
this for example:

(test-multi (map identity [:x])) ;=> java.lang.IllegalArgumentException: No method in multimethod 'test-multi' for dispatch value (:x) of class clojure.lang.LazySeq

This patch addresses this issue by formatting the dispatch value via pr-str and printing the class before it is passed to the exception constructor. The same is also done for the methods in MultiFn.java that throw a dispatch value as part of their exception message.

Answer 1

Comment made by: bronsa

What if the value is infinite lazy-seq?

Answer 2

Comment made by: r0man

Nicola, I forgot those. But I think infinite sequences could be handled with:

(set! *print-length* 10)

I'll try it out and will update the patch later.

Any other edge cases in mind?

Answer 3

_Comment made by: r0man_

After having read "Controlling run-away trains, onions, and exercise
bikes" [1] I now bind *print-length* and *print-size* when building
the error message. This helps when not being able to dispatch on this
for example:


(test-multi (let [x (atom 0)] (reset! x {:deeper x})))


However I'm not sure if this helps in the following case, where
dispatching would fail on an infinite seq.


(test-multi (iterate inc 0))


The above doesn't terminate in Clojure 1.6.0, nor does it when binding
*print-length* like the attached patch does.

[1] http://blog.n01se.net/blog-n01se-net-p-85.html

Answer 4

Reference: https://clojure.atlassian.net/browse/CLJ-1394 (reported by r0man)