4.5: Block Syntax

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Blocks (lexical closures) provide a mechanism to defer the execution of expressions. A block is essentially an anonymous function with a definition context. A block is executed by sending it the message value. The block answers the value of the last expression in its body, unless there is an explicit return (with ^) in which case it returns the value of the returned expression.

[ 1 + 2 ] value
>>> 3

[ 3 = 3 ifTrue: [ ^ 33 ]. 44 ] value
>>> 33

Blocks may take parameters, each of which is declared with a leading colon. A vertical bar separates the parameter declaration(s) from the body of the block. To evaluate a block with one parameter, you must send it the message value: with one argument. A two-parameter block must be sent value:value:, and so on, up to 4 arguments.

[ :x | 1 + x ] value: 2
>>> 3
[ :x :y | x + y ] value: 1 value: 2
>>> 3

If you have a block with more than four parameters, you must use valueWithArguments: and pass the arguments in an array. (A block with a large number of parameters is often a sign of a design problem.)

Blocks may also declare local variables, which are surrounded by vertical bars, just like local variable declarations in a method. Locals are declared after any arguments:

[ :x :y |
    |z|
    z := x + y.
    z ] value: 1 value: 2
>>> 3

Blocks are actually lexical closures, since they can refer to variables of the surrounding environment. The following block refers to the variable x of its enclosing environment:

|x|
x := 1.
[ :y | x + y ] value: 2
>>> 3

Blocks are instances of the class BlockClosure. This means that they are objects, so they can be assigned to variables and passed as arguments just like any other object.