Error Handling and Debugging in Racket | Schema Programming Part 7

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.