# Difference between revisions of "Reverse Compose"

 `⍛`

Reverse compose (`⍛`), also known as Before, is a primitive operator closely related to Compose (`∘`), also known as After. 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 `{(⍺⍺ ⍺) ⍵⍵ ⍵}`. Reverse compose was introduced in Extended Dyalog APL, and then adopted into dzaima/APL.

## 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:

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```      3×⍨⍛-2
7
```

It can also be combined with Compose to create the split-compose construct. Here, we take the sign of the left argument and apply it to the absolute value of the right argument:

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```      3 ¯1 4×⍛×∘|¯2 ¯7 1
2 ¯7 1
```
APL built-ins 
Primitive functions
Scalar