Reverse Compose
⍛
|
Reverse Compose (⍛
) or Behind is a primitive operator closely related to Beside (∘
), which appears in Extended Dyalog APL and dzaima/APL. Called dyadically with function operands f
and g
, it uses f
monadically to pre-processes the left argument before applying g
between the pre-processed left argument and the given right argument. X f⍛g Y
is thus equivalent to (f X) g Y
. The operator can be defined as the dop {(⍺⍺ ⍺) ⍵⍵ ⍵}
. This dyadic definition matches the hook function Before, represented as ⊸
in BQN.
Unlike Before, the monadic case of Reverse Compose has differed across implementations. When introduced by Extended Dyalog APL, f⍛g Y
evaluated to g Y
, but the later Dyalog APL Vision defines[1] it to be (f Y) g Y
, matching Before. This later definition might also be written f⍛g
f⍛g⍨⍨
g⍨∘f⍨
. In dzaima/APL the monadic case is simply an error.
Common usage
Its plain usage is to pre-process left arguments without needing one or more applications of Commute (⍨
). For example, the square of the left argument minus the right argument can be expressed as:
3×⍨⍛-2 7
It can also be combined with Beside to create the split-compose construct. Here, we take the sign of the left argument and apply it to (that is, multiply it with) the absolute value of the right argument:
3 ¯1 4×⍛×∘|¯2 ¯7 1 2 ¯7 1
External links
Documentation
- BQN (as
⊸
)
Publications
References
- ↑ Brudzewsky, Adám. Dyalog APL Vision. JotUnderbar.