Classic Edition: the symbol ⍤ is not available in Classic Edition, and the Atop operator is instead represented by ⎕U2364.

f can be any monadic function. Y can be any array that is suitable as the right argument to function g with the result of g being appropriate to function f.

If X is omitted, g must be a monadic function. If X is specified, g must be a dyadic function and X can be any array that is suitable as the left argument to function g.

The derived function is equivalent to fgY or fXgY and need not return a result.

The Atop operator allows functions to be glued together to build up more complex functions. For further information, see Function Composition.

Examples:

      -⍤÷ 4      ⍝ (  f⍤g y) ≡  f   g y
¯0.25
      12 -⍤÷ 4   ⍝ (x f⍤g y) ≡ (f x g y)
¯3
      3 1 4 1 5 ~⍤∊ 1 2 3
0 0 1 0 1