Monadic operator

In APL syntax, a monadic operator is an operator with one operand, which is written to the operator's left. Outer Product is usually also considered a monadic operator, even though it is written with two symbols and the operand on the right. A specified axis may also be considered a monadic operator, even though its syntax is anomalous: it is written with brackets, and can apply to the Reduce and Scan operators even though operators are not usually allowed to be operands.

The term "monadic operator" refers to the valence of the operator itself, that is, the number of operands. When applied, it produces a derived function, which can have a different function valence. For example, the 2-wise reduction  illustrates the monadic operator Reduce  applied to a single operand , and then to two arguments   and. We say it is a "monadic operator called dyadically".