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Computing with ioLProgrammer's GuideLanguage Reference

abs tag   add tag   align field   and tag   array tag   audio tag   backgroundColor field   bold field   border field   borderBottom field   borderColor field   borderLeft field   borderRight field   borderStyle field   borderTop field   bottom field   box tag   button tag   carat field   checkBox tag   choiceBox tag   class tag   clear tag   clientFrame tag   color field   console tag   dialog tag   div tag   do field   ellipse tag   else field   eq tag   false tag   focus field   focusOrder field   font field   for tag   ge tag   gt tag   height field   hotKey field   if tag   img tag   inf tag   inline field   input tag   invalid tag   int tag   italic field   join tag   layer field   le tag   left field   limit field   line tag   lt tag   margin field   marginBottom field   marginLeft field   marginRight field   marginTop field   mod tag   mul tag   ne tag   neg tag   not tag   null tag   num tag   onClear field   onClick field   onCommit field   onContextClick field   onExit field   onFocus field   onGet field   onKeyDown field   onKeyPress field   onKeyUp field   onMouseDown field   onMouseIn field   onMouseMove field   onMouseOut field   onMouseUp field   onPull field   onPush field   onResize field   onTimer field   openLink tag   or tag   pivot field   playing field   put tag   putError tag   putLn tag   requestTransfer tag   right field   rotate field   scalar tag   scroll field   scrollPos field   selectable field   spacing field   spacingBottom field   spacingLeft field   spacingRight field   spacingTop field   span tag   split tag   sub tag   table tag   tabRow tag   then field   timer tag   top field   transfer tag   transferAbility tag   transition field   true tag   underline field   vector tag   video tag   visible field   void tag   width field   with field   xor tag


null tag

Category: ephemeral logic tag

Produces or tests for the value null.

Sample usage

<null> — produces the value null.

<null {}> — Tests the value {} and produces false since although an empty string is equal to null, it is not the null value itself.

Fields

Primary field

Category: vector field

Optional. The primary field may contain one or more values to test against.

Behavior

The null tag, when used without any field values, may be used to express the literal value null anywhere it is required.

When at least one primary field value is specified, the tag functions in a different way: it tests values for being exactly the value null.

This is different from using the eq tag to check if a value is 'equal' to null. Many values such as 0, false, or {} are evaluated as being equal to null, but the null tag tests for an exact match to the value null itself.

This is an important distinction. A value of null may mean input has not been provided, while a value of {} may mean the user has deliberately provided blank input. Explicitly testing for null allows your program to tell the difference.

Example

The following code displays a dialog box to prompt the user to enter their name. If the user enters a blank name, the code assigns {Player 1} as the user's name. However, if the user has entered a non-blank string of text, or simply cancelled the dialog box (indicated by an explicit null value), the value of the user's name is left unchanged.

<name:dialog text=true {Please enter your name:}>
<if <not|or name,<null name>> then=
    <name {Player 1}>
>

The condition in the above if tag can be deconstructed as follows: