System Labels: Difference between revisions
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<p>Normally, execution of | <p>Normally, execution of an [[Anonymous_Functions/Operators/Hyperators|Anonymous Function/Operator/Hyperator]] (AFOH) or User-Defined Function/Operator/Hyperator (UDFOH) starts execution at line one. However, in certain contexts, execution may start at a different place depending upon the context and the presence of a <b>System Label</b>. These special labels start with a quad symbol so as to distinguish them from normal labels. The following table lists the system labels defined so far and the special context in which they are used.</p> | ||
<table border="1" cellpadding="5" cellspacing="0" rules="none" summary=""> | <table border="1" cellpadding="5" cellspacing="0" rules="none" summary=""> | ||
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<table border="0" cellpadding="5" cellspacing="0" summary=""> | <table border="0" cellpadding="5" cellspacing="0" summary=""> | ||
<tr> | <tr> | ||
<td valign="top"><apll> | <td valign="top"><apll>⎕ID:</apll></td> | ||
<td></td> | <td></td> | ||
<td></td> | <td></td> | ||
<td>is called | <td>is called when an identity element is needed as in <apll>foo/R</apll> where <apll>R</apll> is empty.</td> | ||
</tr> | </tr> | ||
<tr> | <tr> | ||
<td valign="top"><apll> | <td valign="top"><apll>⎕MS:</apll></td> | ||
<td></td> | |||
<td></td> | |||
<td>is called when an AFOH/UDFOH is invoked by the [[Multisets|Multiset Operator]].</td> | |||
</tr> | |||
<tr> | |||
<td valign="top"><apll>⎕PRO:</apll></td> | |||
<td></td> | <td></td> | ||
<td></td> | <td></td> | ||
<td>is called | <td>is called when a prototype element is needed as in <apll>foo¨R</apll> where <apll>R</apll> is empty, or <apll>L foo¨R</apll> where one of both of <apll>L</apll> or <apll>R</apll> is empty and the other is conformable.</td> | ||
</tr> | </tr> | ||
</table> | </table> | ||
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</table> | </table> | ||
<br /> | <br /> | ||
<p>For example,</p> | <p>For example, as a UDFOH</p> | ||
<apll><pre> | |||
∇ Z←{L} foo R | |||
[1] Z←L,R ⋄ →0 | |||
[2] ⎕ID :Z←'Identity' ,(⎕NC 'L'),(⎕NC 'R') ⋄ →0 | |||
[3] ⎕MS :Z←'Multiset' ,(⎕NC 'L'),(⎕NC 'R') ⋄ →0 | |||
[4] ⎕PRO:Z←'Prototype',(⎕NC 'L'),(⎕NC 'R') ⋄ →0 | |||
∇ | |||
</pre></apll> | |||
or equivalently as an AFOH | |||
<apll><pre> | |||
foo←{ | |||
<span style="color:blue;">➥</span>0:⍺←0 ⋄ ⍺,⍵ | |||
<span style="color:blue;">➥</span>⋄ ⎕ID :'Identity' ,(⎕NC '⍺'),⎕NC '⍵' | |||
<span style="color:blue;">➥</span>⋄ ⎕MS :'Multiset' ,(⎕NC '⍺'),⎕NC '⍵' | |||
<span style="color:blue;">➥</span>⋄ ⎕PRO:'Prototype',(⎕NC '⍺'),⎕NC '⍵'} | |||
⎕FMT foo/⍬ | |||
┌──────────────┐ | |||
│┌8───────────┐│ | |||
││Identity 0 2││ | |||
│└────────────┘2 | |||
└∊─────────────┘ | |||
{⍺+÷⍵ ⋄ ⎕ID:∞}/⍬ | |||
∞ | |||
{⍺+÷⍵ ⋄ ⎕ID:∞}\7⍴1 | |||
1 2 1.5 1.666666667 1.6 1.625 1.615384615 | |||
foo⍦ 1 | |||
Multiset 0 2 | |||
2 foo⍦ 1 | |||
Multiset 2 2 | |||
⎕FMT 1 foo¨⍬ | |||
┌0──────────────┐ | |||
│┌11───────────┐│ | |||
││ 0 0││ | |||
│└─────────────┘2 | |||
└∊──────────────┘ | |||
foo/¨0⍴⊂⍬ | |||
NONCE ERROR | |||
foo/¨0⍴⊂⍬ | |||
∧ | |||
</pre></apll> | |||
<apll> | * <p>In the Identity element case, the AFOH/UDFOH is called with the Reduction function's right argument '''prototype''' as the right argument to the AFOH/UDFOH; the left argument is undefined. For example, in <apll>foo/3 0⍴⊂⍳4</apll>, the (right) argument passed to the <apll>⎕ID:</apll> entry point is <apll>0 0 0 0</apll>. The return value from the AFOH/UDFOH is used as the common item in the result. Thus, the <apll>⎕ID</apll> entry point is called only once even though the result may have multiple copies of the return value.</p> | ||
< | * <p>In the Multiset case, the AFOH/UDFOH is called with the same argument(s) as the Multiset derived function.</p> | ||
* <p>In the | * <p>In the Prototype case, the AFOH/UDFOH is called with arguments that are the '''prototypes''' of the respective arguments; in the monadic case, the left argument is undefined.</p> | ||
* <p> | * <p>For the moment, if two or more system labels would be called at the same time (such as the last example above), a <apll>NONCE ERROR</apll> is signaled.</p> | ||
* | * This idea was taken from the paper by David A. Rabenhorst, "[http://portal.acm.org/citation.cfm?id=801228 APL function variants and system labels]", ACM SIGAPL APL Quote Quad, APL83, Volume 13, Issue 3 (March 1983), pp. 281-284. |
Revision as of 14:08, 19 March 2019
Normally, execution of an Anonymous Function/Operator/Hyperator (AFOH) or User-Defined Function/Operator/Hyperator (UDFOH) starts execution at line one. However, in certain contexts, execution may start at a different place depending upon the context and the presence of a System Label. These special labels start with a quad symbol so as to distinguish them from normal labels. The following table lists the system labels defined so far and the special context in which they are used.
|
For example, as a UDFOH
∇ Z←{L} foo R [1] Z←L,R ⋄ →0 [2] ⎕ID :Z←'Identity' ,(⎕NC 'L'),(⎕NC 'R') ⋄ →0 [3] ⎕MS :Z←'Multiset' ,(⎕NC 'L'),(⎕NC 'R') ⋄ →0 [4] ⎕PRO:Z←'Prototype',(⎕NC 'L'),(⎕NC 'R') ⋄ →0 ∇
or equivalently as an AFOH
foo←{ ➥0:⍺←0 ⋄ ⍺,⍵ ➥⋄ ⎕ID :'Identity' ,(⎕NC '⍺'),⎕NC '⍵' ➥⋄ ⎕MS :'Multiset' ,(⎕NC '⍺'),⎕NC '⍵' ➥⋄ ⎕PRO:'Prototype',(⎕NC '⍺'),⎕NC '⍵'} ⎕FMT foo/⍬ ┌──────────────┐ │┌8───────────┐│ ││Identity 0 2││ │└────────────┘2 └∊─────────────┘ {⍺+÷⍵ ⋄ ⎕ID:∞}/⍬ ∞ {⍺+÷⍵ ⋄ ⎕ID:∞}\7⍴1 1 2 1.5 1.666666667 1.6 1.625 1.615384615 foo⍦ 1 Multiset 0 2 2 foo⍦ 1 Multiset 2 2 ⎕FMT 1 foo¨⍬ ┌0──────────────┐ │┌11───────────┐│ ││ 0 0││ │└─────────────┘2 └∊──────────────┘ foo/¨0⍴⊂⍬ NONCE ERROR foo/¨0⍴⊂⍬ ∧
In the Identity element case, the AFOH/UDFOH is called with the Reduction function's right argument prototype as the right argument to the AFOH/UDFOH; the left argument is undefined. For example, in foo/3 0⍴⊂⍳4, the (right) argument passed to the ⎕ID: entry point is 0 0 0 0. The return value from the AFOH/UDFOH is used as the common item in the result. Thus, the ⎕ID entry point is called only once even though the result may have multiple copies of the return value.
In the Multiset case, the AFOH/UDFOH is called with the same argument(s) as the Multiset derived function.
In the Prototype case, the AFOH/UDFOH is called with arguments that are the prototypes of the respective arguments; in the monadic case, the left argument is undefined.
For the moment, if two or more system labels would be called at the same time (such as the last example above), a NONCE ERROR is signaled.
- This idea was taken from the paper by David A. Rabenhorst, "APL function variants and system labels", ACM SIGAPL APL Quote Quad, APL83, Volume 13, Issue 3 (March 1983), pp. 281-284.