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In this session, 
I’d like to complete the one

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about the debugger and
show you how to detect a bug.

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Let’s imagine we change
the decrement definition

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and we make a mistake.

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Here, I put an addition instead
of a substraction.

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When I run my tests, the
TestDecrement returns false.

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I execute it, I look at the 
execution and I notice

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that indeed count returns
9 instead of 5.

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So it shows that I do have 
an error among these

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expressions, if I consider that
the counter has been created.

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Here I’d like to execute

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the code and stop just after

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the Counter creation to see
which methods

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have a problem.

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i add Self halt, this is one of
the ways to propose a

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break point in Pharo. I compile
again the method, I do

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Proceed, the code is executed
and stops on the halt method.

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So now I click on “Over”.

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I can decide to go inside

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the count method, that’s
what I do for example,

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I can see that count value is
nil and Integer value is 7

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like here, and there I can click
on “Over”.

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I go back, I have decrement.

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I’m going to look inside it

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because the problem must be there,
and here

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I can see that decrement is 
equal to -1.

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I modify it, “Over”.

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And now I can click on “Proceed”.

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When I click on “Proceed” the debug
is left, I remove it,

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I click on “Proceed” and my test is green.