Tacit programming

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Tacit functions apply to implicit arguments following a small set of rules. This is in contrast to the explicit use of arguments in dfns ( ) and tradfns (which have named arguments). Known dialects which implement trains are Dyalog APL, dzaima/apl, ngn/apl and NARS2000.


All primitive functions are tacit. Some APLs allow primitive functions to be named.

      plus  +
      times  ×
      6 times 3 plus 5


A train is a series of functions in isolation. An isolated function is either surrounded by parentheses or named. Arguments are processed by the following rules:

A 2-train is an atop:

  (g h)   g (  h )
 (g h)   g ( h )

A 3-train is a fork:

  (f g h)   (  f ) g (  h )
 (f g h)   ( f ) g ( h )

The left tine of a fork (but not an atop) can be an array:

  (A g h)   A g (  h )
 (A g h)   A g ( h )

Expressing algorithms

One of the major benefits of tacit programming is the ability to convey a short, well-defined idea as an isolated expression (example).

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