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Hello, everyone.

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Today we're talking about
errors frequently made

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by Pharo developers including myself.

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We'll see how to find and fix
common mistakes faster.

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Here we have a bit of code.

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When it's executed, a debugger opens

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and tells us that the message
"self" has been sent to an object

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and this object doesn't
understand the message "self".

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We might say that "self"
isn't a message that's sent very often,

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so there's probably a mistake
somewhere in the code.

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Taking a little look at it,

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we see there's a period missing,

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and so the execution
is happening as though

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"self" was a message sent
as a result of "DiceHandle new".

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Since the DiceHandle class
doesn't have a "self" method,

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the debugger opens.

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So the solution is to add this period
at the end of the first line.

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Another problem we often see

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is messages that shouldn't theoretically
be combined that are combined.

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So here we have an error saying
that "includes:ifTrue doesn't exist".

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"Includes" exists,"ifTrue" exists,
"Includes:IfTrue" doesn't.

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Looking closer,

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we realise that indeed

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the message is being sent
"includes:ifTrue"

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to receiver "x" with 2 parameters,
"33" and a block,

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and that doesn't work.

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When Pharo sees a key word,

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it tries to see
all the subsequent key words.

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It takes them all
and considers it as one message.

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So what's missing here

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is a pair of parenthesis to say

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that the message "ifTrue" is sent
as a result of "x includes:33".

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In the same way,
"assert:includes does not exist",

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what we wanted to do was
"assert on the result of includes",

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so the parentheses are missing here.

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Don't hesitate to put parentheses
when you have

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multiple keyword messages
in the same expression,

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to delimit them, because Pharo
will try to group keywords together

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and consider it to be one message.

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In this example,

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we want to have a collection
of numbers in Numbers,

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and for the moment,
there's only one number there, 35.

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However, if we see what's in numbers,
its not a collection, it's the number.

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It's the number 35, so there's a problem.

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In the same way, in this code,

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if I send the message "new"
to the Dice class

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I get the number 6

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rather than a 6-sided die.

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It's the same problem in both examples.

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If we look more closely,

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adding "yourself" after "add"

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will correct the problem. Why?

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Because "add" returns its settings.

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So "OrderedCollection new add: 35"

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returns 35.

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If we add the string to "yourself"
we're sure to get the receiver at the end

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and Numbers will be
a collection of numbers.

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So the solution here

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is to add "yourself"
at the end of each message.

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Here's another problem.

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Here we have a Book class
in "Borrow" method.

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When we execute, we get the message

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that "nil does not understand ifFalse".

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So we send the message
"ifFalse" here to nil.

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What does that mean?
It means that in library

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at the value nil, which has
the default value of all the variables,

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we can say that probably
"inLibrary" has never been initialized.

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We have to put a default value
in that variable.

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It's pretty easy to correct

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by adding the method "initialize",

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which from the creation of each
instance of the Book class

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will put the value "True"

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in the instance variable in Library.

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Except that if we execute this code now,

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we'll get another error message,

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"Class True
does not understand ifFalse".

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Where does this come from?
It's because, what we put here,

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is a class.

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It's not a Boolean, it's a class.

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The Boolean is "true" with a small "t".

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Classes generally have a capital letter,

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so "True" with a capital is a class,

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and "true" with a small "t"
is the unique instance of the True class.

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Here's another problem.

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In the "roll" method
in the Dice class we expect,

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when we roll a Dice, to get a number

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between 1 and the number
of faces on the die,

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except that here,
when we roll the die we get a die

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and not the face we landed on.

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The method I just showed you
is equivalent to the method below.

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This means that by default, a method
that returns nothing returns "self".

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This means our "roll" method,
when executed, returns the die

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and not the result of sending "roll"...

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Not the result of sending "atRandom"
to the "faces" collection.

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So the same problem
in a slightly different example.

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Here, we're creating a new method,

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in the Dice class,

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so in Dice class,

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we want to make a new method
to create instances in the Dice class,

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which initializes by default
the number of faces at zero.

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If we send the message "new"
to the Dice class,

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what we'll get is the Dice class itself

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rather than a new instance
of the Dice class.

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So in both cases,

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the fact that there's no return
in "return self"

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and "self" by default is the receiver,
in the case of a class method

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"self" is the class.

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To correct these 2 problems,

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we just have to add the caret ^
to return to a specific value.

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Next problem,

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if this code is executed,

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the system seems to be frozen
and nothing else happens.

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It's impossible to interact with Pharo.

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What causes this problem?

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It comes from the fact that
we're implementing a new method

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in Dice class.

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"Self" is Dice

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and so "self new"
will call itself recursively.

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The intention here

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is to use the creation
of instance by default

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defined in the Dice superclass,

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and then add things
in relation to that.

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By writing like this,
we have an infinite loop,

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so we need to replace
"self" with "super"

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to request the implementation
of the superclass.

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What you should know,

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we all make lots of mistakes.

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The ones I've shown you
are very frequently made

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by all Pharo developers,

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so there are things we find
very frequently:

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missing periods,

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parentheses,

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missing carets ^,

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and "yourself".

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Try to use the debugger
as much as you can to find

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the root of problems.
It will really help you.

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Don't close it as soon as it opens.

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You'll be missing out
on a way to fix problems.