f may be any monadic function. g may be any monadic function which returns a result. Y may be any array whose items are appropriate to function g. The items of gY must be appropriate to function f.
The derived function is equivalent to fgY. The derived function need not return a result.
Composition allows functions to be glued together to build up more complex functions.
RANK ← ⍴∘⍴ RANK ¨ 'JOANNE' (2 3⍴⍳6) 1 2
+/∘⍳¨2 4 6 3 10 21 ⎕VR'SUM' ∇ R←SUM X [1] R←+/X ∇ SUM∘⍳¨2 4 6 3 10 21