Variant: Difference between revisions

Since <syntaxhighlight lang=apl inline>⍠</source> was not in Dyalog's [[atomic vector]] when the feature was added, a system operator alternative was provided in form of <syntaxhighlight lang=apl inline>⎕OPT</source>.

Variant dates back all the way to [[Iverson notation]] where a subscript was used to indicate which [[index origin]] a [[primitive]] would operate under. This system was dropped in [[APL\360]]. Instead, the [[system command]] <syntaxhighlight lang=apl inline>)ORIGIN</source> was provided for controlling the interpreter's global setting. This was then supplemented with a [[system variable]] to control the global state. Though this feature is still present in many APL systems, together with numerous other system variables for controlling other aspects of the system, for example [[comparison tolerance]], these global settings were recognised as an unfortunate design in that every programmer would be forced to set and localise every system variable that could affect the given code. Later, [[Ken Iverson]] envisioned a return to a per-invocation setting, similar to the original notation, but this time in the form of a dyadic operator that would customise [[primitive function]]s.

Although [[SHARP APL]] didn't implement Iverson's idea, [[J]] did. Later [[Dyalog APL 13.0]] (2011) repurposed it as a way of providing auxiliary arguments to [[primitive function|primitive]] and [[system function]]s, and [[derived function|functions derived]] from [[primitive operator|primitive]] and [[system operator]]s, however its only current use in connection with primitives is to provide language settings for the Format Date-time [[I-beam]] (<syntaxhighlight lang=apl inline>1200⌶</source>).<ref>[[Dyalog Ltd]]. Language Reference Guide: [https://help.dyalog.com/latest/#Language/I%20Beam%20Functions/Format%20Datetime.htm Format Date-time].</ref>

The Dyalog system operator <syntaxhighlight lang=apl inline>⎕S</source> takes as operands text to be searched for, and what to return for each match (with <code>&</code> meaning the matched text):

</source>{{Works in|[[Dyalog APL]]}}

Only one match is found, because <syntaxhighlight lang=apl inline>⎕S</source> defaults to being case sensitive and disallow overlapping matches. The variant operator can be used to enable case insensitivity:

</source>{{Works in|[[Dyalog APL]]}}

</source>{{Works in|[[J]]}}

However, Dyalog also allows a [[wikipedia:name–value pair|name–value pair]], or a vector of name–value pairs. The above example can be spelled using the <syntaxhighlight lang=apl inline>IC</source> (for ''Ignore Case'') setting name:

</source>{{Works in|[[Dyalog APL]]}}

Multiple settings can either be specified with multiple applications of variant, or by using a vector of name–value pairs. Here, <syntaxhighlight lang=apl inline>OM</source> (for ''Overlapping Matches'') is enabled in addition to <syntaxhighlight lang=apl inline>IC</source>:

</source>{{Works in|[[Dyalog APL]]}}

* [https://code.jsoftware.com/wiki/Vocabulary/bangdot J] (as <syntaxhighlight lang=j inline>!.</source>)