Editing Rebol (section)

====do====
The most frequently used evaluator is the <code>do</code> function. It is used by default to interpret the text input to the interpreter [[command line interface|console]].

The ''do dialect'' interpreted by the <code>do</code> function, is an [[expression-oriented programming languages|expression-oriented]] sublanguage of the ''data exchange dialect''. The main semantic unit of the language is the [[expression (programming)|expression]]. In contrast to imperative programming languages descending from [[ALGOL]], the ''do dialect'' has neither keywords, nor statements.

Words are used as [[case sensitivity|case-insensitive]] [[variable (programming)|variables]]. Like in all [[dynamically typed language]]s, variables don't have an associated type, type is associated with values. The result, i.e. the evaluation of a word is returned, when a word is encountered by the <code>do</code> function. The set-word form of a word can be used for [[assignment (computer science)|assignment]]. While not having statements, assignment, together with [[side-effect (computer science)|functions with side-effects]] can be used for [[imperative programming]].<ref name="OG" />

Subblocks of the root block evaluate to themselves. This property is used to handle data blocks, for [[structured programming]] by submitting blocks as arguments to [[control flow|control]] functions like <code>if</code>, <code>either</code>, <code>loop</code>, etc., and for dialecting, when a block is passed to a specific interpreter function.<ref name="RD" />

A specific problem worth noting is that composite values, assigned to variables, are not copied. To make a copy, the value must be passed to the <code>copy</code> function.<ref name="OG" />

The <code>do</code> function normally follows a [[polish notation|prefix style of evaluation]], where a function processes the arguments that follow it. However, [[infix notation|infix evaluation]] using infix [[operator (programming)|operator]]s exists too. Infix evaluation takes precedence over the prefix evaluation. For example,
 [[absolute value|abs]] -2 + 3
returns 1, since the infix addition takes precedence over the computation of the absolute value. When evaluating infix expressions, the order of evaluation is left to right, no operator takes [[Order of operations|precedence]] over another. For example,
<syntaxhighlight lang="smalltalk">
2 + 3 * 4
</syntaxhighlight>
returns 20, while an evaluation giving precedence to multiplication would yield 14. All operators have prefix versions. <code>Do</code> usually evaluates arguments before passing them to a function. So, the below expression:
<div class="plainlinks">
 print read http://en.wikipedia.org/wiki/Rebol
</div>
first reads the Wikipedia Rebol page and then passes the result to the <code>print</code> function. Parentheses can be used to change the order of evaluation. Using [[Polish notation|prefix notation]], the usage of parentheses in expressions can be avoided.<ref name="OG" />

The simple precedence rules are both an advantage:
* No need to "consult" precedence tables when writing expressions
* No need to rewrite precedence tables when a new operator is defined
* Expressions can be easily [[transliteration|transliterated]] from infix to prefix notation and vice versa
as well as a disadvantage:
* Users accustomed to more conventional precedence rules may easily make a mistake<ref name="RD" />