function.md
In rswipl: Embed 'SWI'-'Prolog'

class function {#class-function}

A function object is a code object that, when <-execute'd, returns a PCE data object as the virtual machine instruction get(). Functions are evaluated iff:

They appear as part of a code object that is executed
They are passed to the type-checker (see type <-check) and the type does not allow for functions to be passed.

Explicit evaluation is achieved using <-execute and <-forward. Quoting (e.i. delaying evaluation) is achieved using a quote_function object.

PCE currently provides the following functions:

Class ? (obtainer) An obtainer represents a get() operation. It evaluates to the return value of the get operation represented.
Class binary_expression Super-class for the common arithmetic functions: +, -, *, /
Class var A var represents a PCE variable. Among these are the predefined variables @arg1 ..., @receiver, @event, etc.
Class when A when is a conditional function. It may be compared to Lisp's if or C's Cond ? Expr1 : Expr2.
Class progn Sequence of statements closed with a function.

NOTE: Many methods defined on class object reappear on this class. The message passing system will, when the receiver is a subclass of class function, not evaluate the function if the method is defined at the level of class function or lower in the class hierarchy.

@see type<-check @see class code

function<-_execute: -> unchecked Objects of class function itself cannot be executed. Yields an error message and fails. Redefined by all subclasses of class function.
function<-forward: any ... -> unchecked
function<-_forward: any ... -> unchecked Bind @arg1, ... from the arguments and <-execute the function. Note that @arg1 is a function itself, thus
```
?- get(@arg1, forward, 4, X).       ==> X = 4
```
function<-convert: quote=quote_function -> function Converts an instance of class quote_function into the function quoted. See class quote_function for details.
function<-execute: -> unchecked Explicitly evaluate the function. See class function itself for the default evaluation rules. Class quote_function avoids evaluation of function objects.

Calls <-_execute to perform the real job.