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 48
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 ⍵)
One of the major benefits of tacit programming is the ability to convey a short, well-defined idea as an isolated expression (example).