Clojure not catching NumberFormatException

Question

In the following code, Clojure (1.2) is printing the wrong message:

(try
  (let [value "1,a"]
    (map #(Integer/parseInt %) (.split value ",")))
  (catch NumberFormatException _ (println "illegal argument")))

This should print "illegal argument", but instead it prints a (1#<NumberFormatException java.lang.NumberFormatException: For input string: "a">.

What am I doing wrong?

Is this because of the lazy sequence returned by map? How should it be written?

You should find this helpful: groups.google.com/group/clojure/msg/f4ea5a9c38f4d991 — MayDaniel
– MayDaniel, Commented Feb 5, 2011 at 18:22

Brian · Accepted Answer · 2011-02-05 20:25:24Z

10

The try special form only catches exceptions that are raised during during the dynamic extent of the body code. Here map is returning a lazy sequence, which then is passed out of the try special form and returned. The printer then evaluates the sequence, and at that point the exception is thrown.

Wrapping the map in doall should fix your problem.

answered Feb 5, 2011 at 20:25

Brian

1,82212 silver badges9 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

Ralph Over a year ago

I actually tried that. It still seems to return a NFE wrapped in a RuntimeException.

Brian Over a year ago

@Ralph: Under 1.3 it works. You could wrap the parseInt call with try and return some sensible value if an exception occurs. If that is not appropriate, you could return nil when an exception occurs, and check to see if the resulting sequence contains nil to determine if an exception occurred and then act appropriately.

Ralph Over a year ago

The second suggestion (return nil) sounds reasonable. I'll check that out.

Ralph Over a year ago

I tried the nil substitution solution and it worked like a charm. Thanks!

funkotron Over a year ago

This example skips invalid solutions using the above nil substitution method: (let [value "1,a"] (remove nil? (map #(try (Integer/parseInt %) (catch Exception _ nil)) (.split value ","))))

Collectives™ on Stack Overflow

Clojure not catching NumberFormatException

1 Answer 1

5 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

5 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related