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CS 535 Object-Oriented Programming & Design
Fall Semester, 2001
Basic Smalltalk Syntax
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San Diego State University -- This page last updated 04-Sep-01
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Contents of Doc 2, Basic Smalltalk Syntax
References
Object-Oriented
Design with Smalltalk — a Pure Object Language and its Environment,
Ducasse,
University of Bern, Lecture notes 2000/2001,
http://www.iam.unibe.ch/~ducasse/WebPages/Smalltalk/ST00_01.pdf
Smalltalk
Best Practice Patterns, Kent Beck, Prentice Hall, 1997
Basic
Smalltalk Syntax
The
Xerox team spent 10 years developing Smalltalk
They
thought carefully about the syntax of the language
Smalltalk
syntax is
- Different
from other languages
The
Rules
- Everything
in Smalltalk is an object
- All
actions are done by sending a message to an object
- Every
object is an instance of a class
- All
classes have a parent class
Entire
Language ExampleFrom
http://st-www.cs.uiuc.edu/users/johnson/
exampleWithNumber: x
“This is a small method that illustrates every part of Smalltalk method syntax
except primitives, which aren’t very standard. It has unary, binary, and key
word messages, declares arguments and temporaries (but not block temporaries),
accesses a global variable (but not and instance variable), uses literals (array,
character, symbol, string, integer, float), uses the pseudo variable true false,
nil, self, and super, and has sequence, assignment, return and cascade. It has both
zero argument and one argument blocks. It doesn’t do anything useful, though”
| y |
true & false not & (nil isNil) ifFalse: [self halt].
y := self size + super size.
#($ a #a ‘a’ 1 1.0)
do: [: each | Transcript
show: (each class name);
show: (each printString);
show: ‘ ‘].
^ x < y
Sample
Program
"A Sample comment"
| a b |
a :='this is a string'. ":= is assignment"
a := 'this is '' a string that contains
a single quote and a newline'.
a := 'concat' , 'inate'.
a := 5.
a := 1 + "comments ignored" 1.
b := 2 raisedTo: 5.
^a + b "^ means return"
Multiple
Assignments
Assignment
statements return values!
| a b |
a := b := 3 + 4.
a
and b now contain 7
Statement
Separator
| cat dog |
cat := 5.
dog := cat + 2
A
period is used as a statement separator
A
period is optional after the last statement
Identifiers
An
identifier (any name) in Smalltalk is of the form:
- letter
(letter | digit )*
Variables
| cat dog |
cat := 5.
dog := cat + 2.
Vertical
bars at the top of a program declare variables
Variables
must be declared
All
variables are references to objects
Variables
are initialized to nil
As
we will see numbers in Smalltalk are objects. Internally references to objects
require pointers. Always using a pointer to refer to a number would slow
arithemitic operations. Most Smalltalk virtual machines will store numbers
directly in a variable. At the programming level one does not see any
difference in how numbers and other objects are handled.
Messages
Most
languages place basic operations in the grammar
-
- >,
=, for (int k = 1; k < 10; k++)
In
Smalltalk operations are defined as methods in a class
- +
is a method in the Integer class
- In
3 + 4, + is a message sent to the integer 3
Using
messages rather than hard coded grammar makes
Three
type of Messages
-
- 1
+ 2
- 12
/ 6
-
- 12.3
printString
- '123'
asNumber
-
- 'Hi
mom' copyFrom: 1 to: 3
All
messages contain:
- Receiver
- Selector
- Zero
or more arguments
Messages
always return a value
Unary
Messages
Format:
- aReceiver
aSelector
25 factorial
- 25
is the receiver
- factorial
is the selector
- returns
15511210043330985984000000
'this is a string' reversed
- 'this
is a string' is the receiver
- reversed
is the selector
- returns
'gnirts a si siht'
'Cat in the hat' size
- returns
14
12 printString
- returns
‘12’ (a string)
‘20’ asNumber
- returns
20 (an integer)
Combining
Unary Messages
Unary
messages are executed from left to right
100 factorial printString size
is
done as:
((100 factorial) printString) size
How
about this?
100 factorial size
This
will not work
100
factorial returns an integer
Integers
do not implement a size method
One
uses the Smalltalk browser to see what methods a class implements
Binary
Messages
Format:
- aReceiver
aSelector anArgument
2 + 4
- 2
is the receiver
- +
is the selector
- 4
is the argument
- returns
6
Binary
selectors are
- Arithmetic,
comparison and logical operations
- One
or two characters taken from:
-
- +
- / \ * ~ < > = @ % | & ! ? ,
- Second
character is never -
Using
the above rules you can create your own binary messages in Smalltalk. You can
make @? a binary method in a class.
Combining
Binary Messages
Binary
messages are executed from left to right
1 + 2 * 3 * 4 + 5 * 6
is
executed as
((((1
+ 2) * 3) * 4) + 5) * 6
Keyword
Messages
Format:
- receiver
keyword1: argument1 keyword2: argument2 ...
12 min: 6
- 12
is the receiver
- min:
is a selector with only one keyword
- 6
is the argument
- returns
6
'this is a string'
copyFrom: 1
to: 7
- 'this
is a string' is the receiver
- copyFrom:to:
is one selector with two keywords
- 1
and 7 are the arguments
- returns
'this is'
'this
is a string'
findString:
'string'
startingAt:
4
ignoreCase:
true
useWildcards:
false
- 'this
is a string' is the receiver
- findString:startingAt:ignoreCase:useWildcards:
is one selector
- ‘string’,
4, true, false are the arguments
- returns
(11 to: 16)
Keyword
Messages verses Positional Argument ListsSmalltalk Version
'this
is a string'
findString:
'string'
startingAt:
4
ignoreCase:
true
useWildcards:
false
- Each
keyword communicates role of argument
Positional
Argument List Version
'this
is a string'.findString( 'string', 4, true, false);
- More
common so more familiar
- Easy
for compiler to parse
- Easier
for programmer to mix up parameters
Where
do Keyword Messages End?
Unless
you use parenthesis the compiler combines all keywords in a statement into one
message
'this is a string'
copyFrom: 1
to: 12 min: 7
- The
above has one message
- copyFrom:to:min:
- This
message does not exist, so results in an error
'this is a string'
copyFrom: 1
to: (12 min: 7)
- This
message contains two legal keyword messages
Formatting
Keyword Messages
'this is a string'
findString: 'string'
startingAt: 4
ignoreCase: true
useWildcards: false
or
'this is a string' findString: 'string' startingAt: 4 ignoreCase: true useWildcards: false
Beck’s
Rule
When
a keyword message has two or more keywords
- Place
each keyword with its argument on its own line
- Indent
the keyword one tab from the receiver
Program
formatting is a matter of personal preference. Some Smalltalk style guides
state that keyword messages with two keywords should be placed on one line.
Whichever style one uses consistency is very important. Consistent style makes
it easier for others to read your code. When you work on a team, the entire
team should use the same style. Many companies have programming styles fro all
programmers to fallow.
The
Tab Verses Spaces Debate
When
one indents a line of code do you use:
-
- Easier
to type
- Sometimes
tabs are different on screen and on hard copy
- Some
companies ban tabs
Smalltalk
handles tabs uniformly
Use
tabs to indent in Smalltalk
Do
not use spaces to indent in Smalltalk
Precedence
- First
unary messages are parsed left to right
- Binary
messages are parsed left to right after unary messages
- Keyword
messages are parsed after binary messages
Parenthesis
change the order of evaluation
Expression
|
Result
|
3
+ 4 * 2
|
14
|
3
+ (4 * 2)
|
11
|
5
+ 3 factorial
|
11
|
(5
+ 3) factorial
|
40320
|
'12'
asNumber + 2
|
14
|
Arithmetical
operations do not use normal mathematical precedence rules
Parenthesis
must be used to separate multiple keyword messages in one statement
'this is a string' reversed
findString: ('the cat is white' copyFrom: 9 to: 10)
startingAt: 1 + 2
caseSensitive: 2 + 2 = 4
Transcript
Special
output window
Similar
in purpose to Java's System.out and C++'s out
Useful
Transcript messages:
clear
- clear
the Transcript
show: aString
- display
aString in the Transcript
print: anObject
- display
a string representation of anObject in the Transcript
nextPutAll: aString
- add
aString to the display buffer
endEntry
- put
contents of display buffer in Transcript
- empty
the buffer
flush
- Same
as endEntry
tab cr space crtab crtab: anInteger
- put
given character in the display buffer
Sample
Program
Transcript clear.
Transcript show: 'This is a test'.
Transcript cr.
Transcript show: 'Another line'.
Transcript tab.
Transcript print: 12.3.
Transcript cr.
Transcript show: 4 printString.
Transcript cr.
Transcript show: 'The end'.
Result
of Running Program
Cascading
Messages
Format:
- receiver
selector1 [arg] ; selector2 [arg] ; ...
A
cascade sends multiple messages to the same receiver
Messages
are sent from left to right to the same receiver
Transcript
clear;
show:
'This is a test';
cr;
show:
'Another line';
tab;
print:
12.3;
cr;
show:
4 printString;
cr;
show:
'The end'.
Cascade
Versus Compound Messages
Expression
|
Result
|
'hi
mom' reversed asUppercase
|
'MOM
IH'
|
'hi
mom' reversed; asUppercase
|
'HI
MOM'
|
Compound
In
a compound message each message is sent to the result of the previous message
'hi
mom' reversed asUppercase
- First
send reversed to 'hi mom'
- The
result is 'mom ih'
- Now
send asUppercase to 'mom ih'
Cascade
In
a cascade message each message is sent to the same receiver
'hi
mom' reversed; asUppercase
- First
send reversed to 'hi mom'
- The
result is not used
- Now
send asUppercase to 'hi mom'
Copyright ©, All rights reserved.
2000 SDSU & Roger Whitney, 5500 Campanile Drive, San Diego, CA 92182-7700 USA.
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