↢ — After

Keys: Alt-,

f↢g Y is Y f(g Y). The left argument of f is Y itself. For f(g Y), use Atop, f∘g.

×↢- 5              ⍝ ¯25

X f↢g Y is X f(g Y).

10 +↢- 3           ⍝ 7
"abc" ,↢⌽ "de"     ⍝ "abced"

f↢A binds the right argument. The bound function takes one argument. A literal argument after a space joins a literal operand’s strand: -↢10 3 binds the vector 10 3 and has no argument. Use -↢10⊢3, -↢10(3), or a named argument.

-↢10⊢3            ⍝ ¯7
y←3 ⋄ -↢10 y       ⍝ ¯7

See Before, Atop / Rank and Over.

Inverse

With an array operand, (f↢X)⁻¹Y inverts f with X fixed on the right. With two functions, X(f↢g)⁻¹Y inverts f with X fixed, then applies g⁻¹. The hook (×↢*)⁻¹ gives the Lambert W function.

(-↢3)⁻¹7   ⍝ 10
(×↢*)⁻¹1   ⍝ 0.567143290409784