⌿), also called Reduction or Insert, is a primitive monadic operator which takes a dyadic function operand, inserts it between the elements of the argument, and evaluates it into a single array in right-to-left order. This operation is known as Fold, or more specifically <syntaxhighlight lang=text inline>foldr1</source>, in other functional programming languages such as Haskell.
When applied to a vector argument, <syntaxhighlight lang=apl inline>f/x</source> evaluates to the expression <syntaxhighlight lang=text inline>a f b f c f d …</source> where <syntaxhighlight lang=text inline>a, b, c, d, …</source> are the elements of <syntaxhighlight lang=text inline>x</source>. In general, Reduce reduces one chosen axis (either implied by using the last-axis form <syntaxhighlight lang=apl inline>f/</source> or first-axis <syntaxhighlight lang=apl inline>f⌿</source>, or explicitly by using function axis <syntaxhighlight lang=apl inline>f/[x]</source>) by evaluating each vector along the chosen axis into a scalar.
In leading axis model, Reduce only has the first-axis form, and it reduces the major cells of the entire array, not the individual elements. It does not enclose the result either. Instead, reduction over an axis other than the first is performed via the Rank operator, which mixes the results into a flat array.
Reduce is mainly used for aggregation, such as sum (using Add) or product (using Times). If used with Subtract, it computes the alternating sum, since . Using with Divide gives similar effect, returning the alternating product .
+/1 2 3 4 5
×/1 2 3 4 5
-/1 2 3 4 5
÷/1 2 3 4 5
Although Reduce is <syntaxhighlight lang=text inline>foldr1</source> in nature, one can use it like <syntaxhighlight lang=text inline>foldr</source>, where a designated starting value is modified by the rest of the values in sequence. In this case, the start value (enclosed if not a simple scalar) is attached to the right end of the vector of "modifiers", and then the entire vector is reduced.
(⍉∘⌽↓)/2 1 2 1,⊂5 6⍴⍳30 ⍝ Trim a matrix from all four sides, by rotating the matrix after each trim
┌─────┐ │ 9 10│ │15 16│ │21 22│ └─────┘
○/1 ¯2,⊂0 0.25 0.5 0.75 1 ⍝ sin∘arccos of multiple values
┌──────────────────────────────────────────┐ │1 0.9682458366 0.8660254038 0.6614378278 0│ └──────────────────────────────────────────┘ </source>
Reduction over an empty axis gives the identity element of the operand.
+/2 3 0⍴0
0 0 0 0 0 0 </source>
FinnAPL idiom library contains over 100 entries which use Reduce in some way.