4.3. Blocks, Booleans, and Control Structures

A block is a special kind of object containing a sequence of statements. When a block is evaluated by being sent an acceptable value message, its statements are executed in the context of the current activation of the method in which the block is declared. This allows the statements in the block to access variables local to the block’s enclosing method and any enclosing blocks in that method. (This set of variables comprises the lexical scope of the block.) It also means that within the block, self refers to the receiver of the message that activated the method, not to the block object itself. A return statement in a block causes a return from the block’s enclosing method. (See chapter Language Reference - Language Reference for a more thorough discussion of block semantics.)

A block can take an arbitrary number of arguments and can have its own local variables, as well as having access to the local variables of its enclosing method. The statements in the block are executed when the block is sent a message of the form “value[:{With:}]”, where the number of colons in the message is at least the same as the number of arguments the block takes (extra arguments are ignored, but it is an error to provide too few). For example, the following block takes two arguments:

[| :arg1. :arg2 | arg1 + arg2 ]

and can be evaluated by sending it the message value:With: to produce the sum of its arguments. Blocks are used to implement all control structures in Self and allow the programmer to easily extend the system with customized control structures. In fact, all control stuctures in Self except message sends, returns, and VM error handling are implemented using blocks.

4.3.1. Booleans and Conditionals

The fundamental control structure is the conditional. In Self, the behavior of conditionals is defined by two unique boolean objects, true and false. Boolean objects respond to the messages ifTrue:, ifFalse:, ifTrue:False:, and ifFalse:True: by evaluating the appropriate argument block. For example, true implements ifTrue:False: as:

ifTrue: b1 False: b2 = ( b1 value )

That is, when true is sent ifTrue:False:, it evaluates the first block and ignores the second. For example, the following expression evaluates to the absolute value of x:

x < 0 ifTrue: [ x negate ] False: [ x ]

The booleans also define behavior for the logical operations AND (&&), OR (||), EXCLUSIVE-OR (^^), and NOT (not). Because the binary boolean operators all send value to their argument when necessary, they can also be used for “short-circuit” evaluation by supplying a block, e.g.:

(0 <= i) && [i < maxByte pred] ifTrue: [...

Module: boolean

4.3.2. Loops

The various idioms for constructing loops in Self are best illustrated by example. Here is an endless loop:

[ ... ] loop

Here are two loops that test for their termination condition at the beginning of the loop:

[ proceed ] whileTrue: [ ... ]
[ quit ] whileFalse: [ ... ]

In each case, the block that receives the message repeatedly evaluates itself and, if the termination condition is not yet met, evaluates the argument block. The value returned by both loop expressions is nil.

It is also possible to put the termination test at the end of the loop, ensuring that the loop body is executed at least once:

[ ... ] untilTrue: [ quit ]
[ ... ] untilFalse: [ proceed ]

Here is a loop that exits from the middle when quit becomes true:

[| :exit | ... quit ifTrue: exit ... ] loopExit

For the incurably curious: the parameter to the user’s block, supplied by the loopExit method, is simply a block that does a return from the loopExit method. Thus, the loop terminates when exit value is evaluated. The constructs loopExitValue, exit, and exitValue are implemented in a similar manner.

The value returned by the overall [...] loopExit expression is nil. Here is a loop expression that exits and evaluates to a value determined by the programmer when quit becomes true:

[| :exit | ... quit ifTrue: [ exit value: expr ] ] loopExitValue

Module: block

4.3.3. Block Exits

It is sometimes convenient to exit a block early, without executing its remaining statements. The following constructs support this behavior:

[| :exit | ... quit ifTrue: exit ... ] exit
[| :exit | ... quit ifTrue: [ exit value: expr ] ... ] exitValue

The first expression evaluates to nil if the block exits early; the second allows the programmer to define the expression’s value when the block exits early. Note: These constructs should not be confused with their looping counterparts loopExit and loopExitValue.

Module: block

4.3.4. Other Block Behavior

Blocks have some other useful behavior:

  • One can determine the time in milliseconds required to execute a block using various ways of measuring time using the messages userTime, systemTime, cpuTime, and realTime.

  • One can profile the execution of a block using the messages profile and flatProfile. profile prints out the source level call graph annotated with call site and timing information whereas flatProfile prints out a flat profile sorted by module.

  • The message countSends will collect lookup statistics during a block execution.

Any object that inherits from the lobby can be passed to a method that expects a block; behavior in defaultBehavior makes the object behave like a block that evaluates to that object.

Module: block