Reverse Compose

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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 fg 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:

Try it online!

      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:

Try it online!

      3 ¯1 4××|¯2 ¯7 1
2 ¯7 1
APL built-ins [edit]
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Primitive operators Monadic EachReplicate
Dyadic Reverse Compose
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