Prototype: Difference between revisions

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Miraheze>Adám Brudzewsky
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Miraheze>Adám Brudzewsky
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{{Built-ins|Prototype|∊|⊤}} is a [[monadic function]] which in the [[nested array model]] gives information about the type and structure of its argument array's [[element]]s. The result is derived from the first element of the array in [[ravel order]], or, if the array is [[Empty array|empty]], from information stored alongside the array (usually this information is just the prototype itself). An array's prototype is used as a [[fill element]] for operations involving that array, and to determine some [[identity element]]s.
The '''prototype''' of an array is, in the [[nested array model]], an array which gives information about the type and structure of its [[element]]s. It is derived from the first element of the array in [[ravel order]], or, if the array is [[Empty array|empty]], from information stored alongside the array (usually this information is just the prototype itself). An array's prototype is used as a [[fill element]] for operations involving that array, and to determine some [[identity element]]s.


== Support==
== Support ==
Few dialects have Prototype as a [[primitive function]]. [[Dyalog APL]] uses <source lang=apl inline></source> (as in ''prototypical '''e'''lement''), but only when [[migration level]] is 0, while [[NARS2000]] uses [[monadic]] <source lang=apl inline></source> (resembling the Latin letter '''T''' for '''''T'''ype'').
A few dialects have [[Type]] as a [[primitive function]], and in those, the prototype is simply the type of the first element:
 
<source lang=apl>
However, an arrays prototype can always be determined by enclosing the array in an empty array, and then coercing our a fill value. Since prototypes are used as fill elements, the resulting array will be the prototype of the original array:
      ∊⊃ 1 2 'ab'
┌─┬─┬──┐
│0│0│  │
└─┴─┴──┘
</source>
{{Works in|[[Dyalog APL]] with [[migration level]]<nowiki>=</nowiki>0}}
<source lang=apl>
      ⊤↑ 1 2 'ab'
┌─┬─┬──┐
│0│0│  │
└─┴─┴──┘
</source>
{{Works in|[[NARS2000]]}}
However, an array's prototype can always be determined by enclosing the first element in an empty array, and then coercing out a fill value:
<source lang=apl>
      ⊃ 0 ⍴ ⊂ 1 2 'ab'
┌─┬─┬──┐
│0│0│  │
└─┴─┴──┘
</source>
{{Works in|[[Dyalog APL]] with [[migration level]]<nowiki>=</nowiki>1}}
<source lang=apl>
<source lang=apl>
       ⊃ 0 ⍴ ⊂ 1 2 'ab'
       ⊃ 0 ⍴ ⊂ 1 2 'ab'
Line 11: Line 31:
└─┴─┴──┘
└─┴─┴──┘
</source>
</source>
{{Works in|[[Dyalog APL]] with [[migration level]]<nowiki>≥</nowiki>2, [[APL2]], [[APLX]]}}


APLs with a [[flat array model]] do not typically store prototype information, instead using only the array's type ([[character]], [[numeric]], or [[Box|boxed]]) to determine its [[Fill element|fills]].
APLs with a [[flat array model]] do not typically store prototype information, instead using only the array's type ([[character]], [[numeric]], or [[Box|boxed]]) to determine its [[Fill element|fills]].
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{{Works in|[[Dyalog APL]]}}
{{Works in|[[Dyalog APL]]}}
{{APL programming language}}
{{APL programming language}}
{{APL built-ins}}

Revision as of 15:53, 13 November 2019

The prototype of an array is, in the nested array model, an array which gives information about the type and structure of its elements. It is derived from the first element of the array in ravel order, or, if the array is empty, from information stored alongside the array (usually this information is just the prototype itself). An array's prototype is used as a fill element for operations involving that array, and to determine some identity elements.

Support

A few dialects have Type as a primitive function, and in those, the prototype is simply the type of the first element:

      ∊⊃ 1 2 'ab'
┌─┬─┬──┐
│0│0│  │
└─┴─┴──┘
Works in: Dyalog APL with migration level=0
      ⊤↑ 1 2 'ab'
┌─┬─┬──┐
│0│0│  │
└─┴─┴──┘
Works in: NARS2000

However, an array's prototype can always be determined by enclosing the first element in an empty array, and then coercing out a fill value:

      ⊃ 0 ⍴ ⊂ 1 2 'ab'
┌─┬─┬──┐
│0│0│  │
└─┴─┴──┘
Works in: Dyalog APL with migration level=1
      ⊃ 0 ⍴ ⊂ 1 2 'ab'
┌─┬─┬──┐
│0│0│  │
└─┴─┴──┘
Works in: Dyalog APL with migration level≥2, APL2, APLX

APLs with a flat array model do not typically store prototype information, instead using only the array's type (character, numeric, or boxed) to determine its fills.

Examples

The prototype is used when an array is expanded or when empty elements are inserted in some other way. Take and Expand on a character array produce an array with spaces, because ' ' is the prototype of a character array:

      ¯12 ↑ 'whitespace'
  whitespace
      (5 1 5/1 ¯2 1) \ 'whitespace'
white  space

The Rank operator combines its left operand's result cells by extending them to a common shape like Mix does. Numeric vectors of different lengths will be expanded with 0, the prototype for a simple numeric array.

      (⍳⍤0) 2 3 4
1 2 0 0
1 2 3 0
1 2 3 4
Works in: Dyalog APL

An array's prototype is based on its first element, so the array a below has a simple numeric prototype, as does any prefix of a. This property even applies to empty arrays: the expression 0 ↑ a results in an empty array with the same prototype as a.

      ⊢a ← 5 'c' ¯2
5 c ¯2
      4 ↑ a
5 c ¯2 0
      4 ↑ 2 ↑ a
5 c 0 0
      4 ↑ 0 ↑ a
0 0 0 0
Works in: Dyalog APL

The prototype of an array may be nested. Here, we inspect the prototype of an array containing both character and numeric data. In the prototype, all characters are converted to spaces and all numbers to zeros. Thus the prototype retains type and structure information but not specific values.

      ⊢x ← ⊂'de'(3 4 5)   ⍝ A complicated scalar array
┌──────────┐
│┌──┬─────┐│
││de│3 4 5││
│└──┴─────┘│
└──────────┘
      0 1 \ x             ⍝ Its fill element is nested
┌──────────┬──────────┐
│┌──┬─────┐│┌──┬─────┐│
││  │0 0 0│││de│3 4 5││
│└──┴─────┘│└──┴─────┘│
└──────────┴──────────┘
      e ← 0 ↑ x           ⍝ An empty array based on x
      ⊃e                  ⍝ Disclosing gets the prototype
┌──┬─────┐
│  │0 0 0│
└──┴─────┘
Works in: Dyalog APL

Template:APL programming language