Re: errors?

[Stephen C. Gilardi]

On Mar 13, 2009, at 7:50 PM, Raoul Duke wrote:

i'm up to date with the clojure jar. the error messages i get seem
awfully terse and not particularly helpful in learning what i'm doing
wrong. for example, with the code below, when i try (bs 0 (vector 1 2
3)) i get "java.lang.ClassCastException:
clojure.lang.LazilyPersistentVector (NO_SOURCE_FILE:0)". swapping the
order of args in the 2nd 'nth' call fixes it.  is there no way to get
Clojure to help me out a little with such things?

There are two contribs that can help make finding the problem easier.





Clojure
1:1 user=> (defmulti bs (fn [find data] (count data)))
#'user/bs
1:2 user=> (defmethod bs 0 [find data] -1)
#<MultiFn clojure.lang.mult...@4d668534>
1:3 user=> (defmethod bs 1 [find data]
 (if
     (= (nth data 0) find)
   0
   -1))
#<MultiFn clojure.lang.mult...@4d668534>
1:8 user=> (defmethod bs :default [find data]
 (let [lo 0
        hi (count data)
        mid (/ (+ lo hi) 2)
        testv (nth mid data)]
   -42))
#<MultiFn clojure.lang.mult...@4d668534>
1:14 user=> (bs 0 (vector 1 2 3))

1:15 user=> (use 'clojure.contrib.stacktrace)
nil
1:16 user=> (e)

 at clojure.core/nth (core.clj:892)
    user/fn (:12)
    clojure.lang.MultiFn.invoke (MultiFn.java:152)
    user/eval (:14)
    clojure.lang.Compiler.eval (Compiler.java:4522)
    clojure.core/eval (core.clj:1738)
    clojure.main$repl__5665$read_eval_print__5677.invoke (main.clj:176)
    clojure.main$repl__5665.doInvoke (main.clj:193)
nil
1:17 user=>








--Steve

So just to be clear, you found the error in your code and now you'd like to know if Clojure could have pointed you to it more quickly, right? First, using Java 1.6 would help some. It gives both the source and destination types for a class cast error: java.lang.ClassCastException: clojure.lang.LazilyPersistentVector cannot be cast to java.lang.Number (NO_SOURCE_FILE:0) Clojure lets you see the stack trace associated with an exception at the repl. When the repl catches an exception, it binds it to the var *e. You can see its stack trace with (.printStackTrace *e). That stack trace includes a lot of Java-level stuff that can make it more difficult to parse visually than it has to be. Here's a session using clojure.contrib.stacktrace and clojure.contrib.repl_ln to point to the line of the error: % java -cp clojure.jar:../clojure-contrib/clojure-contrib.jar clojure.contrib.repl_ln java.lang.ClassCastException: clojure.lang.LazilyPersistentVector cannot be cast to java.lang.Number (repl-1:14) java.lang.ClassCastException: clojure.lang.LazilyPersistentVector cannot be cast to java.lang.Number clojure.contrib.repl_ln provides (by default) the line number as part of the prompt. clojure.contrib.stacktrace provides the "e" convenience function that prints a more readable version of the stack trace that's part of *e. Looking at the output of (e), you can see that your call to "nth" is the problem and that you're calling it on the 12th input line from this repl which (counting down from line 8) is the line that begins "testv". From there, (doc nth) and looking at how its argument list(s) match up with your code leads to the fix.

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