Error Handling and Debugging in Racket | Schema Programming Part 7

Code on GitHub: All examples from this tutorial are available at racket-programming-series/07-error-handling — clone the repo and follow along!

Even the best programmers make mistakes. Racket provides tools to catch, handle, and fix these errors gracefully.

Raising Errors

You can intentionally signal an error using error.

(define (divide a b)
  (if (= b 0)
      (error "Cannot divide by zero!")
      (/ a b)))

Handling Exceptions

To catch errors, use with-handlers. It works like try/catch in other languages.

(with-handlers ([exn:fail? (lambda (exn) 
                             (display "Oops, something went wrong!")
                             0)])
  (divide 10 0))
; Output: Oops, something went wrong!
; Returns: 0

• exn:fail?: A predicate checking if the exception is a failure.
• lambda (exn) ...: The handler code.

Defensive Programming with Contracts

Racket goes beyond simple asserts with its Contract System. You can define exactly what inputs a function expects.

(define/contract (multiply-positive x y)
  (-> positive? positive? positive?)
  (* x y))

(multiply-positive 5 10) ; Works
; (multiply-positive -5 10) ; Auto-generates a detailed error message

Debugging Tips

1. DrRacket Debugger: DrRacket has an excellent graphical debugger. Click the "Debug" button to step through code, hover over variables to see values, and analyze stack traces.
2. printf Debugging: If all else fails, printing values is a classic.

   (printf "Current value of x: ~a\n" x)
   

3. Rackunit: Racket's built-in unit testing framework.

   (require rackunit)
   (check-equal? (+ 2 2) 4 "Simple addition")
   

Summary

Robust software anticipates failure. Racket's exception system mixed with its powerful contract system allows you to build highly reliable applications.

Next step: Organizing large programs with Modules.