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3.5: Block Syntax

  • Page ID
    36346
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    Blocks provide a mechanism to defer the evaluation of expressions. A block is essentially an anonymous function. A block is evaluated 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 \(\uparrow\)), in which case it does not answer any value.

    [1+2]value → 3
    

    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 BlockContext. This means that they are objects, so they can be assigned to variables and passed as arguments just like any other object.

    Caveat: In the current version (3.9), Squeak does not actually support true block-closures, since block arguments are actually simulated as temporary variables of the enclosing method. There is a new compiler that supports full block closures, but it is still being worked on and is not used by default. In somewhat obscure situations this problem can lead to naming conflicts. This situation arises because Squeak is based on an early implementation of Smalltalk. If you encounter this problem, have a look at the senders of the method fixTemps, or load the Closure Compiler.


    This page titled 3.5: Block Syntax is shared under a CC BY-SA 3.0 license and was authored, remixed, and/or curated by Andrew P. Black, Stéphane Ducasse, Oscar Nierstrasz, Damien Pollet via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.