Debugging in Pharo

What You Will Learn

• The system is alive: Communicate with it
• The debugger is your best friend
• Don't be afraid of it

Debugging

Debugging

• Closing the debugger does not solve bugs
• The debugger is your best friend
  • communicate with objects of the context
  • check state
  • send messages to specific objects
  • compile code on the fly
  • continue without restarting from scratch

Watch the videos and practice

Simple Trace

Transcript show: 'x = ', x printString

• used when you don't have tools
• often inefficient
• we can do better

Defining a Breakpoint

   ...
   Halt now.

Halt now (or self halt)

• pause the program
• invoke the debugger

Single-Shot Halt

  ...
  Halt once.

To enable it, evaluate

  Halt enableHaltOnce

Halt once, if enabled :

• pauses the program
• opens a debugger
• disables itself

Halt After n Iterations

Halt onCount: 10

Conditional Halt

• if: aSelector stops when invoked from a aSelector
• if: aBlock stops if the block evaluates to true

faces will stop only when invoked from printString

Dice >> faces
  ...
  Halt if: #printString

Conditional Halt

The parameter passed to if: can be a test name too:

Dice >> faces
  ...
  Halt if: #testLargeDice

faces will stop only when invoked from testLargeDice

Create Your Own Breakpoints

• now, once, onCount: and if: are methods in Halt class
• you can add your own methods, e.g.,

Halt class >> between: minTime and: maxTime
   (Time current
      between: minTime asTime
      and: maxTime asTime)
         ifTrue: [ self signal ]

Dice >> faces
   ...
   Halt between: '00:00' and: '02:00'

faces will halt only between midnight and 2am.

What You Should Know

• The debugger is a powerful tool
• You should communicate with objects
• Breakpoints are powerful and customizable