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Operator vs. function test
(Operator vs. function test) |
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</source>{{Works in|[[APL2]], [[Dyalog APL]], [[APLX]], [[ngn/APL]], [[GNU APL]]}} | </source>{{Works in|[[APL2]], [[Dyalog APL]], [[APLX]], [[ngn/APL]], [[GNU APL]]}} | ||
=== Operator or function? === | |||
The syntax <source lang=apl inline>a / b</source> is ambiguous: it may be an invocation of a [[dyadic function]] <source lang=apl inline>/</source> with left [[argument]] <source lang=apl inline>a</source> and right argument <source lang=apl inline>b</source>, or of a [[monadic operator]] with [[operand]] <source lang=apl inline>a</source> and right argument <source lang=apl inline>b</source>. In early APLs there was no way to resolve this ambiguity, but with the extension of [[operator]]s to allow arbitrary function operands instead of a specified set of [[primitive function]]s, the distinction becomes apparent: a function Replicate can be used as an [[operand]] while an operator Replicate cannot. | |||
One test of Replicate's nature is to try Replicate [[Each]]<ref>Benkard, J. Philip. [https://dl.acm.org/doi/10.1145/384282.28345 "Replicate each, anyone?"]. [[APL87]].</ref> with an expression such as <source lang=apl inline>1 3 /¨ 'ab' 'cd'</source>. If Replicate is implemented as an operator, it will be applied to the operand <source lang=apl inline>1 3</source>, and Each will be applied to the resulting [[derived function]] <source lang=apl inline>1 3/</source>. | |||
<source lang=apl> | |||
1 3 /¨ 'ab' 'cd' | |||
abbb cddd | |||
(1 3/)¨ 'ab' 'cd' | |||
abbb cddd | |||
</source>{{Works in|[[SHARP APL]] (with <source lang=apl inline>¨></source> in place of <source lang=apl inline>¨</source>), [[APL2]], [[APLX]]}} | |||
If Replicate is a function, then Each will apply to Replicate only, and the resulting derived function will be invoked monadically. | |||
<source lang=apl> | |||
1 3 /¨ 'ab' 'cd' | |||
ab cccddd | |||
1 3 (/¨) 'ab' 'cd' | |||
ab cccddd | |||
</source>{{Works in|[[NARS2000]], [[Dyalog APL]], [[GNU APL]]}} | |||
In early APLs such as [[APL\360]], applying an operator to Compress will always result in a [[SYNTAX ERROR]], because Compress is not an allowed operand of any operator. This is also the case in [[ngn/APL]]: although operators can apply to any function, Replicate cannot be used unless both arguments are immediately available. In both cases there is no way to determine whether Replicate "acts like a function" or "acts like an operator". | |||
== History == | == History == | ||