WEBVTT

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Hi. During this class
I'll be showing you exceptions,

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which are really powerful in Pharo.
I'll give you an overview.

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What can we do with exceptions?
We can resume them,

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restart them,
signal them as new exceptions.

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I'm only going to show you
things that will be useful in a practical way

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when first using exceptions,

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but it's important that you know
that this isn't everything.

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There are 2 kinds of exceptions,
"Errors", which are often fatal

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and "notifications"
which we can say we don't need,

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they're just for reference.

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So, if you're interested in reading

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a complete reference on exceptions

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there's a whole chapter in the book,
"Deep Into Pharo",

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which is on the MOOC website
and available free online.

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So, what are you going to learn?
To raise and trap exceptions,

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and some nice helper methods
we can use every day.

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So basically,

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everything about this class
is on this page.

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You're going to install a handler
to trap an exception,

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you'll signal them,

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and we'll have 2 nice messages,
which are "ensure" and "ifCurtailed"

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So, let's take a look.

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Imagine that I wanted
to trap an exception.

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It's the "on: do:" message.

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I make a block, if I've an action,
when I have an action,

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if ever I have an exception,
what do I do?

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"Something" and here is the exception,
which is passed in argument.

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If we look at an example
of zero division,

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if I do 7 divided by 0...

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when I have a zero divide,
what do I have to do?

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I'll write, "Watch out,
there's a zero divide."

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The thing to notice here is that
an instance of the Exception class

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has been passed as an argument,

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and I'll be able to question it
and send it messages,

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because exceptions
are objects in Pharo.

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I'll send messages in my exceptions

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to do something and it will show me 0.

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So how do we raise an exception?

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Basically, we create an exception
and we send it the signal message.

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So here, when we want to give
a warning I do, "Warning new".

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I create an exception, customise it,
I put the message I want inside,

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and send the Signal message.

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When I've an exception
to be able to ask the author,

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writing the method,

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then you'll have another API,

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because it's "AuthorNameRequest"
that defines this API,

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so I've passed "Stef"
then I'll do "signal".

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What usually happens
is that the classes

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propose shortcuts to avoid
having to create, so I'm sending

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"OutOfMemory", that's the class.
I send the signal message to the class.

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Normally, with warnings,
I don't do "Warning new, blah, blah..."

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I do "Warning signal" with the text,
because it's shorter,

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but that depends on your exceptions.

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So, since we like agile
programming with unit tests,

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and test-driven programming,

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how can we test that an exception
is raised in SUnit?

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We're going to use "shouldn't raise"
and "should raise".

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Let's read this together.

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I want to say here,
"Look out, I don't want

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"an exception to be raised,

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"for it to be a SubscriptOutOfBounds.

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"if I create a date in February".

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Now, if I want to test
that I should have an error

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when I create a date in the 13th month,

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something you don't usually get,
in your pay,

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then I'll use "should raise",

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which means, "Watch out,

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"if you create a date in a 13th month,
I'll raise an error".

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That allows you to create the unit test,

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while controlling exceptions.

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It's a digression, but it's important

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to know there are these
2 little helper methods.

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So, what are exceptions in Pharo?

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There are errors:

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"message not understood",
"subscript", "division by zero",

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you have "halt", which we'll look at
when we come to debugging.

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You can put a break point,

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which will stop
execution of the system.

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You have notifications,
everything non fatal,

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so a deprecation,
"don't use this method, use another",

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warnings and timedout.

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One nice thing is,
you have an error message

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which describes unhandled errors.

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This is what raises the debugger.

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We don't normally use it,
but it's just to tell you

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that unhandled errors
are also a class of error.

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Exceptions are real objects.

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If we look at how the
"doesNotUnderstand" message

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is defined, you can see it
in ProtoObject with your Pharo browser,

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you do, "messageNotUnderstood new",

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I send a message to the receiver,
which is the object, and give a signal.

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You get this signal when you send
"point new strangeAndBizarre".

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It's not known as a message,
"strangeAndBizarre"

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so it will create this exception.

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We have deprecation,
for example, which means,

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if you use an API
which evolves over time

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and which Pharo has decided
is no longer valid,

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in fact...

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How do we do that in Pharo?
We say, "deprecated: on: in"

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Here, I've chosen a clear example,

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"menu, item, title"
and what does it say?

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"Use the method addTitle
instead of "title".

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How is it implemented?

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It's implemented
in the following way: "deprecated",

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this, "create an exception",

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which is called "deprecation"

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within which I'll pass it
"method, explanation, on, in",

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which will give a signal.
So it's just to show you that

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"deprecation", "messageNotUnderstood",
are exceptions we create and raise.

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So this is just a detail,
you can have exception sets.

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You can say, "If I have ZeroDivide,
and I have Warning,

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or I have Warning,
you're going to be able,

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to contain the exceptions with a comma",
which will create an exception set,

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allowing you to say,
"I want to react to all these exceptions".

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It's very rarely used,
it's just for completeness.

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Now we're going to look at
some very handy methods,

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which we often use.

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How can I ensure that
an expression is always executed,

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even if the program fails beforehand?

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So the idea is "ensure".
What does "ensure" do?

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You do something,

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and I guarantee that the other block
will be executed.

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It's very powerful,
it lets you to close files automatically.

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That's what we see in SpyOn,
for the Pharo profiler.

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There's a class on it.
I say, " I'm starting profiling",

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I execute the block I want to profile

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and I guarantee myself
I'll stop profiling,

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even if you've passed me
a block with an error.

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There's another helper

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which is really nice,
which is how can I ensure

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an expression is executed only
if the program fails?

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What I'll do is I'll say,
"You do something if ever

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"something strange happens,
I want you to execute this code".

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Here I've taken an example
in "wait", which is really

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very low level, which allows you
to change the processor scheduling.

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I say, "There's the system,
schedule a semaphore,"

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I do an operation and if ever
something doesn't work,

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the system guarantees

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that I'll remove it from the scheduler.

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These methods are really handy,
because they're very powerful.

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I've given you a slide to explain
how the method lookup works,

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in addition to the book.

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Now...

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just for your information,

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the things we can do
with exceptions in Pharo,

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you can return an alternative result,
you can retry, in fact.

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You can change the execution
of the program and say,

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"Act as though
the exception didn't happen".

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I can resume,

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I can say, "No, I don't want to deal
with this exception at this level,

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"I want to deal with it higher up".

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I can resignal an exception,

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"No, you raised this exception
but it's old, I want a new one"

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with resignal.

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When you need this it means
you know what you're doing,

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and you've read the chapter,
so just be aware that it exists.

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This was just to show you.

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I want to return another value
from an exception, I do "return",

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and you can see that here,
I had my block which gave me this value.

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I raised an exception, and in the end,

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it's this value that was returned.
That's with "return".

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Now, with "resume",
what you're going to have,

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"resume" is...

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I've my exception which is raised here,

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and what I see is that,
in my exception, when I do "resume",

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it's not this value that's returned,
it's the next value, so I say,

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"Continue after the exception point",
so start again from here,

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and that will give me this value.

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So now if we look at
what you should know:

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exceptions are powerful in Pharo,
OK, great.

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There's a very simple API,
it's "signal" to raise an exception,

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"on: do:" to trap them,

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and you have "ensure" to guarantee
that this expression is always executed,

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if I have a problem here or not,

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and there's this expression,
with "ifCurtailed",

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which will execute "onProblem"
if I've a problem in "doSomething".

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That's all you need to know
to get started with exceptions.