Editing Smalltalk (section)

===Code blocks===
A block of code (an anonymous function) can be expressed as a literal value (which is an object, since all values are objects). This is achieved with square brackets:

<syntaxhighlight lang="smalltalk">[ :params | <message-expressions> ]</syntaxhighlight>

Where '':params'' is the list of parameters the code can take. This means that the Smalltalk code:

<syntaxhighlight lang="smalltalk">[:x | x + 1]</syntaxhighlight>

can be understood as:

:<math>f(x) = x + 1</math>

or expressed in lambda terms as:

:<math>\lambda x.x + 1</math>

and

<syntaxhighlight lang="smalltalk">[:x | x + 1] value: 3</syntaxhighlight>

can be evaluated as

:<math>f(3) = 3 + 1</math>

Or in lambda terms as:

:<math>(\lambda x. x + 1)\,3 \underset{\beta}\rightarrow 3+1</math>

The resulting block object can form a [[closure (computer programming)|closure]]: it can access the variables of its enclosing lexical scopes at any time. Blocks are [[first-class object]]s.

Blocks can be executed by sending them the ''value'' message. Compound variations exist to provide parameters to the block e.g., <code>value:value:</code> and <code>valueWithArguments:</code>.

The literal representation of blocks was an innovation which on the one hand allowed certain code to be significantly more readable; it allowed algorithms involving iteration to be coded in a clear and concise way. Code that would typically be written with loops in some languages can be written concisely in Smalltalk using blocks, sometimes in a single line. But more importantly blocks allow control structure to be expressed using messages and [[Polymorphism (computer science)|polymorphism]], since blocks defer computation and polymorphism can be used to select alternatives. So if-then-else in Smalltalk is written and implemented as

<syntaxhighlight lang="smalltalk">expr ifTrue: [statements to evaluate if expr] ifFalse: [statements to evaluate if not expr]</syntaxhighlight>
''True methods for evaluation''
{{pre|'''ifTrue:''' trueAlternativeBlock '''ifFalse:''' falseAlternativeBlock<br />
 &nbsp;&nbsp;&nbsp;&nbsp;^trueAlternativeBlock value}}

''False methods for evaluation''
{{pre|'''ifTrue:''' trueAlternativeBlock '''ifFalse:''' falseAlternativeBlock<br />
&nbsp;&nbsp;&nbsp;&nbsp;^falseAlternativeBlock value}}

<syntaxhighlight lang="smalltalk">positiveAmounts := allAmounts select: [:anAmount | anAmount isPositive]</syntaxhighlight>

This is related to [[functional programming]], wherein patterns of computation (here selection) are [[Abstraction (computer science)|abstracted]] into [[higher-order function]]s. For example, the message ''select:'' on a Collection is equivalent to the higher-order function [[Filter (higher-order function)|filter]] on an appropriate [[function object|functor]].<ref>{{cite book|last1=Goldberg|first1=Adele|author-link1=Adele Goldberg (computer scientist)|last2=Robson|first2=David|year=1989|title=Smalltalk-80 The Language|publisher=Addison Wesley|isbn=0-201-13688-0|pages=17–37}}</ref>