1
00:00:00,560 --> 00:00:04,040
This course explores
another advanced feature.

2
00:00:04,200 --> 00:00:07,080
I want to show you how we can transform

3
00:00:07,280 --> 00:00:10,560
the execution stack into an object,
on demand.

4
00:00:11,200 --> 00:00:15,600
The aim is to give you intuitive tools
without going into details.

5
00:00:17,080 --> 00:00:19,840
We can manipulate
and navigate the stack,

6
00:00:20,040 --> 00:00:23,160
and modify it,
but we'll focus on navigating.

7
00:00:23,600 --> 00:00:28,400
There are two chapters
you can read on this subject.

8
00:00:28,640 --> 00:00:31,640
They are the chapters on blocks

9
00:00:31,840 --> 00:00:35,080
and exceptions
in the book <i>Deep into Pharo</i>.

10
00:00:35,280 --> 00:00:39,280
They're worth reading
as they give clear explanations.

11
00:00:39,520 --> 00:00:43,400
In Pharo, the Context class
also represents the stack.

12
00:00:44,600 --> 00:00:45,600
So,

13
00:00:46,280 --> 00:00:50,160
we'll use a scenario
that is already in your debugger.

14
00:00:50,400 --> 00:00:55,000
Here, a message you sent is not found
because you didn't define it,

15
00:00:55,200 --> 00:00:57,520
while you execute a unitary test.

16
00:00:57,840 --> 00:01:02,720
In the debugger, you answer
that you want to create a method.

17
00:01:02,920 --> 00:01:06,040
It compiles and installs it
in the class.

18
00:01:06,200 --> 00:01:07,920
Then it reexecutes it.

19
00:01:08,080 --> 00:01:12,560
This method raises an exception
since the system is not magic!

20
00:01:12,720 --> 00:01:16,200
It sends you
the shouldBeImplemented message

21
00:01:16,360 --> 00:01:19,440
telling you to edit
the method in the debugger.

22
00:01:19,640 --> 00:01:21,440
You implement the method,

23
00:01:21,840 --> 00:01:26,000
you recompile the method on the spot,
then select "proceed"

24
00:01:26,600 --> 00:01:28,080
to resume the program.

25
00:01:28,920 --> 00:01:32,360
There are two key points
in this scenario.

26
00:01:32,920 --> 00:01:36,280
Firstly, we recompile
the method on the fly.

27
00:01:36,640 --> 00:01:40,080
Several times,
but that's the compiler's job.

28
00:01:40,920 --> 00:01:44,920
Secondly, and importantly,
we modify the execution stack

29
00:01:45,080 --> 00:01:48,840
in order to inject new chunks of stack,

30
00:01:49,040 --> 00:01:53,200
which allow us to continue the program
after an error.

31
00:01:53,440 --> 00:01:58,600
It's not just a matter of reifying
the stack, or making it an object.

32
00:01:58,760 --> 00:02:03,640
It's not just a formal exercise,
it can enhance user experience

33
00:02:03,920 --> 00:02:05,560
and create new tools.

34
00:02:05,720 --> 00:02:10,640
It's also used in Seaside
to utilize web applications.

35
00:02:11,080 --> 00:02:15,720
Pharo usually has a C stack,
the virtual machine's own stack.

36
00:02:16,040 --> 00:02:19,360
On demand, we can turn this
into a live object.

37
00:02:19,520 --> 00:02:25,440
What's interesting is that we can
navigate and modify this object.

38
00:02:25,760 --> 00:02:30,200
By modify, I mean that
when we change this Pharo object,

39
00:02:30,400 --> 00:02:34,400
it will change the implicit C stack,
so it's very powerful.

40
00:02:35,800 --> 00:02:41,280
It also supports all exceptions,
so I recommend reading those chapters.

41
00:02:41,560 --> 00:02:44,080
We navigate the stack for exceptions

42
00:02:44,440 --> 00:02:47,920
to search for block catches,

43
00:02:49,920 --> 00:02:51,800
known as exception handlers.

44
00:02:52,000 --> 00:02:57,200
In addition, this ability
to transform the stack into an object

45
00:02:57,400 --> 00:02:59,840
allows us to create continuations

46
00:03:00,040 --> 00:03:04,400
and web servers, such as for
functional languages like Scheme.

47
00:03:05,320 --> 00:03:09,080
To explain how this works,
we can look at the variable

48
00:03:09,200 --> 00:03:13,640
called thisContext,
one of Pharo's three pseudovariables.

49
00:03:14,160 --> 00:03:17,360
These are self, super, and thisContext.

50
00:03:17,920 --> 00:03:22,600
When you ask for thisContext's value,
it returns the execution stack.

51
00:03:22,920 --> 00:03:27,400
That's what you see
when you open the debugger.

52
00:03:27,640 --> 00:03:31,840
The execution stack is displayed,
based on thisContext.

53
00:03:32,200 --> 00:03:36,280
As an exercise, you can define a method

54
00:03:36,400 --> 00:03:39,360
in which you insert "halt."

55
00:03:39,520 --> 00:03:44,400
The debugger will pop up,
but you can also type "thisContext"

56
00:03:45,440 --> 00:03:50,040
and an inspector will open
on the execution stack itself.

57
00:03:50,600 --> 00:03:53,800
Now I'll show you two examples

58
00:03:53,920 --> 00:03:56,400
of how we use thisContext.

59
00:03:56,680 --> 00:04:02,360
The first is deprecation,
used when we want to change API.

60
00:04:02,640 --> 00:04:08,560
As a programmer, we edit the method
and use the message "deprecated"

61
00:04:08,720 --> 00:04:13,640
with "on" and "in," as I explained
in the course on exceptions.

62
00:04:14,680 --> 00:04:19,760
Here we want to express
the message "Use bar."

63
00:04:20,080 --> 00:04:23,840
And what will the deprecation display?

64
00:04:24,000 --> 00:04:26,840
It will show the user the message

65
00:04:27,040 --> 00:04:30,200
"Message foo is deprecated in Pharo."

66
00:04:30,360 --> 00:04:33,920
It's important to note that as a user,

67
00:04:34,200 --> 00:04:36,600
I did not declare the method used,

68
00:04:36,840 --> 00:04:40,920
but the system identified foo
as the method caller.

69
00:04:42,080 --> 00:04:46,640
As you can see, we don't use foo
in the arguments of deprecated

70
00:04:47,320 --> 00:04:48,920
or in the method caller.

71
00:04:49,080 --> 00:04:51,840
So, how is this implemented?

72
00:04:52,600 --> 00:04:54,280
The message "deprecated"

73
00:04:55,920 --> 00:04:58,040
is an exception, "Deprecation."

74
00:04:58,200 --> 00:05:03,160
The system takes the arguments,
anExplanationString, and so on,

75
00:05:03,320 --> 00:05:07,800
and adds the expression
"thisContext sender method."

76
00:05:08,160 --> 00:05:13,920
ThisContext is the stack at the time
of execution of the deprecated method.

77
00:05:15,720 --> 00:05:19,320
Now, using sender
we can access the method caller.

78
00:05:20,000 --> 00:05:23,200
That will give us foo,
which was our example.

79
00:05:23,600 --> 00:05:25,560
Then we ask for the method.

80
00:05:25,720 --> 00:05:29,800
ThisContext sender method
returns the compiled method,

81
00:05:29,920 --> 00:05:31,640
which is an object,

82
00:05:31,800 --> 00:05:35,080
named A>>foo.

83
00:05:35,280 --> 00:05:39,720
So, the exception uses what it needs
to be able to extract

84
00:05:39,920 --> 00:05:43,840
the method selector
and create a clearer message.

85
00:05:44,280 --> 00:05:49,600
It's used to make messages
easier for users to understand,

86
00:05:49,800 --> 00:05:54,040
without forcing the programmer
to hard-code the message source.

87
00:05:54,320 --> 00:05:59,040
Now I'll show you
another very powerful function.

88
00:06:01,640 --> 00:06:05,280
Often when debugging,
we want to insert a breakpoint

89
00:06:05,400 --> 00:06:08,760
in a method that is heavily used.

90
00:06:09,360 --> 00:06:13,320
You may simply want to debug
your version of the program

91
00:06:13,440 --> 00:06:15,720
without halting the whole system.

92
00:06:15,920 --> 00:06:21,040
Conditions like "halt once"
will interrupt the system once,

93
00:06:21,200 --> 00:06:25,680
but what you want
is to halt execution only if the method

94
00:06:25,920 --> 00:06:29,000
has been called by another method.

95
00:06:29,640 --> 00:06:32,840
How does a programmer express this?

96
00:06:33,040 --> 00:06:37,400
We express that we only want to halt
if foo has been called

97
00:06:37,600 --> 00:06:40,520
by the method testSetInitialized.

98
00:06:41,520 --> 00:06:44,280
How do we implement this?

99
00:06:44,440 --> 00:06:47,200
Usually, this method must not be halted.

100
00:06:47,760 --> 00:06:52,760
You can open the code in Pharo
to see how it's implemented.

101
00:06:52,920 --> 00:06:54,840
So, halt throws an exception

102
00:06:55,400 --> 00:06:57,640
by showing the message "If:."

103
00:06:57,840 --> 00:07:00,160
We have several argument options.

104
00:07:00,320 --> 00:07:05,080
In a case involving a symbol,
we pose a query to confirm it's true.

105
00:07:05,320 --> 00:07:10,200
We look to see if the call chain
contains the symbol.

106
00:07:10,400 --> 00:07:11,400
Let's check.

107
00:07:12,360 --> 00:07:15,360
Let's look at the method that comes up.

108
00:07:16,000 --> 00:07:19,800
Suppose that we have a test
that is named

109
00:07:20,840 --> 00:07:22,200
testSetInitialized.

110
00:07:23,800 --> 00:07:25,280
It's an argument here.

111
00:07:26,200 --> 00:07:31,400
First, the method will retrieve
the execution stack, or context,

112
00:07:32,720 --> 00:07:35,560
which is a synonym for execution stack.

113
00:07:35,760 --> 00:07:41,080
So we use Context to access the stack,
then we add whileFalse.

114
00:07:41,200 --> 00:07:44,920
Note that we're not at the top
of the execution stack,

115
00:07:45,160 --> 00:07:48,080
where there is no sender invoking us.

116
00:07:48,440 --> 00:07:52,280
At the top of the stack,
the sender is nil.

117
00:07:52,600 --> 00:07:56,160
In a case where the sender is not nil,

118
00:07:56,520 --> 00:07:59,400
we move between sections of the stack.

119
00:07:59,560 --> 00:08:02,920
We can picture
the areas of stack like this,

120
00:08:03,200 --> 00:08:06,040
and we travel upwards using sender,

121
00:08:06,840 --> 00:08:07,920
each time.

122
00:08:09,080 --> 00:08:12,080
Now we need to see if the selector...

123
00:08:12,200 --> 00:08:15,520
Somewhere in this area,
we should see the test

124
00:08:16,680 --> 00:08:18,320
asking the question

125
00:08:18,840 --> 00:08:23,080
if this stack's call symbol
matches where we intend to halt.

126
00:08:23,200 --> 00:08:26,400
We ask if we have been called
by testSet.

127
00:08:26,640 --> 00:08:31,200
If so, we stop and mark a signal
because the class is an exception.

128
00:08:32,000 --> 00:08:36,320
It's important to understand
that this is difficult to implement

129
00:08:36,440 --> 00:08:39,760
in a language
that does not reify the stack.

130
00:08:40,080 --> 00:08:44,640
Here you see it in five lines,
even if it seems hard to understand.

131
00:08:44,840 --> 00:08:49,560
It's compact and powerful,
and only possible through reification.

132
00:08:50,160 --> 00:08:55,640
ThisContext is rarely used
as it is an advanced feature.

133
00:08:55,920 --> 00:08:59,160
But it is very important
for new innovations,

134
00:08:59,320 --> 00:09:02,080
such as tools, as shown by these tests,

135
00:09:02,840 --> 00:09:05,560
which are facilitated by this language.

136
00:09:05,720 --> 00:09:09,080
It's also used
to represent continuations.

137
00:09:09,400 --> 00:09:12,600
Seaside's creator used it
in Pharo's ancestor

138
00:09:12,760 --> 00:09:16,280
to manipulate the stack
to represent continuations.

139
00:09:16,400 --> 00:09:21,840
This is the basis of the call-and-answer
mechanism in Seaside.

140
00:09:22,280 --> 00:09:27,280
Here you have an advanced feature
of Pharo to play around with.