Tacit programming

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.

Primitives

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

      plus ← +
      times ← ×
      6 times 3 plus 5
48

Trains

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).