Learn to interpret error messages, use `*e` for exceptions, and leverage debugging tools like `clojure.tools.trace` in Clojure's REPL.
As you dive deeper into developing with Clojure, effectively handling errors and debugging becomes crucial to maintaining productivity and software quality. In this section, you’ll learn how to interpret Clojure’s error messages, access exceptions directly in the REPL, and utilize powerful tools like clojure.tools.trace to gain insight into your code’s execution path.
Error messages are an essential part of understanding what went wrong in your code. Clojure’s error messages typically provide valuable information, such as the error type and location. Familiarizing yourself with common error patterns will enable you to quickly diagnose and solve issues. Here’s an example of a typical error message:
;; Example of an error in Clojure REPL
(+ 1 "two")
This will output something like:
Execution error (ClassCastException) at user/eval153 (REPL:1).
java.lang.String cannot be cast to java.lang.Number
*e VariableClojure provides a special variable, *e, which holds the last exception that was thrown. This is especially useful when you’re experimenting in the REPL and need to inspect the details of an error:
;; Access the last exception
(e)
;; Example output
#object[clojure.lang.Compiler$CompilerException 0x1020304 "clojure.lang.Compiler$CompilerException: ..."]
clojure.tools.trace for DebuggingThe clojure.tools.trace namespace can be a powerful ally when debugging your Clojure code. By tracing function calls and their return values, you gain a clearer picture of how data flows through your application. To do this, you need to include the dependency in your project. Here is a simple example:
(require '[clojure.tools.trace :as trace])
(trace/trace (+ 1 2)) ;; Traces the function call and prints the execution steps
traceConsider the following code snippet that contains an error you want to investigate:
(defn faulty-func [x]
(+ x (/ 10 0))) ; Deliberate error for illustration
(trace/trace
(faulty-func 5))
Running the above trace will show you each step leading up to the error, enabling you to identify precisely where the problem occurs.
Explore more with these tools and techniques as you further hone your debugging skills in Clojure, leading to more efficient and effective coding practices.