Summary: We continue our exploration of the Io programming
language.
Prerequisites: The first Io lab and the second Io lab. Sections 3.4-3.5 of Tate.
Contents:
a. Create a directory for the lab.
b. Verify that Io is installed on the workstation you're using by typing
io or /usr/local/bin/io at the command line. If
Io is not installed on your workstation, you can ssh to church, where it
is installed.
The following method is intended to print out information on a message.
Add it to your Io environment. (I would suggest that you save it in a
file with a name like callInfo.io and use launchFile
to load it.
Object callInfo := method(
"Message: " print;
call message println;
"Arguments: " print;
call message arguments println;
"Sender: " print;
call sender println;
"Target: " print;
call target println;
)
a. How do you expect the Io interpreter to respond to the command Object callInfo?
b. Check your answer experimentally.
c. How do you expect the Io interpreter to respond to the command 3 callInfo? (Specify message, parameters, source, and target.)
d. Check your answer experimentally.
e. How do you expect the Io interpreter to respond to the command 3 callInfo(3, "hello", :goodbye)? (Specify message, parameters, source, and target.)
f. Check your answer experimentally.
g. How do you expect the Io interpreter to respond to the command callInfo (that is, with no object specified)?
h. Check your answer experimentally.
i. Summarize what you've learned from this exercise.
Change the name of the slot from the previous command from callInfo
to forward.
a. What do you expect the output of 3 forward to be?
b. Check your answer experimentally.
c. What do you expect the output of 3 foo to be?
d. Check your answer experimentally.
e. What do you expect the output of foo to be?
f. Check your answer experimentally.
g. As you may have just discovered, the change to the Object
message forward method does not seem to affect the interpreter opject. Fill
in the forward slot of the interpreter object.
This exercise is based on an example from Tate that is taken from an
example originally by Jeremy Tregunna and updated by Chris Kappler.
One of the characteristics of Io is that it's easy to build new syntax.
Let's start with a simple : operation, which is intended to
mean store in a hash table
.
a. Add the handler for : with the following command.
OperatorTable addAssignOperator(":", "colon")
b. What do you expect to happen when you try to evaluate 3:4?
c. Check your answer experimentally.
d. As you've noted, the colon method gets called, but we have not defined
that method. So let's define a preliminary version. Define a method,
colon, that prints out its arguments. Then verify that it
works okay with something like "Sam":"x4410".
e. When you printed out the arguments, you may have noticed that
the evaluation of the colon method includes the quotation marks as
part of the first string argument. We want to remove those marks.
Write a method, stripQuotes, that takes a string as
a parameter and returns the string with the opening quotation mark
and closing quotation mark removed.
Note that you'll need to make the string mutable with asMutable
and that you can remove a prefix with removePrefix and a
suffix with removeSuffix.
f. Of course, our real goal is to use the colon operator to fill in
values in a Map object. Add a colon handler
to Map that calls atPut appropriately.
g. Verify that the following works as you might expect:
info := Map clone
info "SamR" : "x4410"
This exercise is based on an example from Tate that is taken from an
example originally by Jeremy Tregunna and updated by Chris Kappler.
a. What do you expect to happen when you type { "Hello" }?
b. Check your answer experimentally.
c. As you've noted, when you type an expression involving curly
brackets, the interpeter object receives a curlyBrackets
message. Define that message to print out its arguments.
d. Check what the output of the following is:
{
"SamR" : "x4410",
"Reg I. Strar" : "x3450"
}
d. As you may recall, for the phone book example, we'd like to evaluate
the "Name" : "Value" pair for each such pair.
Write a curlyBrackets handler for the Map object
that calls doMessage with each argument.
Try the actors example from Tate.
Consider the following method:
countdown := method (list(5,4,3,2,1) foreach(x, x println))
a. What do you expect to happen when you type countdown?
b. Check your answer experimentally.
c. What do you expect to happen when you type @@countdown?
d. Check your answer experimentally.
e. As you may have noted, you got no output. Why? Because the top level
command finished before all of the actors. As the example suggests, you
need to wait a bit to give the actors time to work. What do you expect
to get when you type
Io> @@countdown; wait(0.5)
f. Check your answer experimentally.
g. You may have gotten a very strange answer the first time you tried this
command. Try it again. And again. Have you figured out what's going on
yet?
h. Revise the method to read
countdown := method (list(5,4,3,2,1) foreach(x, wait(0.2); x println))
What do you expect to get when you run the following?
Io> @@countdown; wait(0.5)
i. Check your answer experimentally. Then check it again. Explain
what's happening
This exercise is based on an example from Tate.
As we noted earlier, by intercepting the forward message,
we can alter Io's behavior on undefined messages. We can change the
effect of forward on all objects or simply on objects in
a particular class (or lots of variants thereof).
a. Write an Io class
, Builder0, with a
forward handler that (i) prints out the name of the message
and (ii) prints out the arguments to the message, one per line.
b. Write an Io class
, Builder1, with a
forward handler that (i) prints out the name of the message
and (ii) recurses on the arguments. That is, it should send each argument
to itself with self doMessage(arg).
c. Write an Io class
, Builder2, that works
like Builder1 except that it checks the return value of
doMessage and if it is non-nil, prints it out.
d. Write an Io class
, Builder3, that (i) prints
the message name as an opening tag, (ii) recurses on the arguments
as in Builder2, and (iii) prints the message name as
a closing tag.
Sunday, 6 February 2011 [Samuel A. Rebelsky]
Monday, 7 February 2011 [Samuel A. Rebelsky]
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