Null object: Difference between revisions

=={{header|TXR}}==

=={{trans|Common Lisp}}==

TXR Lisp has a <code>nil</code> symbol which serves as the empty list and Boolean false, like Common Lisp and similar dialects. It is a very important symbol which plays a central role.

Variable definitions, global and local, without initial value expressions take on the value <code>nil</code>. Elements of newly created vectors and structure slots are <code>nil</code> by default. Optional function parameters that don't receive arguments are given <code>nil</code> arguments by default. Many functions which search for something use <code>nil</code> for indicating not found.

Object-oriented programming in TXR Lisp is done with structures, which do not provide a CLOS-like object system. The <code>nil</code> object isn't a structure, and so it cannot take slot references, or method invocation. Only structures have methods, which means that it's not possible to define a null object method <code>m</code> such that <code>obj.(m)</code> can be invoked if <code>obj</code> is an expression evaluating to <code>nil</code>.

The Gang-of-Five Null Object Pattern can be employed: defining struct types that behave like null. This null object doesn't have to be related by inheritance to the structs in conjunction with which it is used.

<syntaxhighlight lang="txrlisp">

(defstruct null-widget ()

(:method popularity (me) 0))

(defvarl null-widget (new null-widget))

(defstruct real-widget ()

pop

(:method popularity (me) me.pop))</syntaxhighlight>

In situations when a null object would be used simply to provide safe treatment of null without the verbosity of checks for its presence, and a default value of <code>nil</code> is acceptable, TXR Lisp has null-safe slot and method access: <code>obj.?slot</code>, <code>obj.?(method arg)</code>.

The expression <code>w.?(popularity)</code> will evaluate to <code>nil</code> if <code>w</code> is <code>nil</code>, being equivalent to <code>(if w w.(popularity))</code>, except that <code>w</code> is evaluated only once.

Like in Common Lisp, <code>nil</code> an instance of the type <code>null</code>, which is a unit type having only that instance. And <code>nil</code> is also the name of the bottom type of the type system: the <code>nil</code> type is the subtype of every type, including itself.

Like in Common Lisp, the expression <code>(or a b c ...)</code> evaluates the arguments from left to right, returning the value of the leftmost one which does not evaluate to <code>nil</code>.

<syntaxhighlight lang="txrlisp">;; Find widget in one of three places in order

(defun find-widget (name)

(or [%widget-hash% name]

(lookup-local-widget name)

(lookup-global-widget name)))</syntaxhighlight>