In Reactive Programming, operators take one or several input streams and return a new stream. Some operators may accept additional arguments, such as functions:

• project function,
• predicate function,
• accumulator function, etc.

These functions don’t have to work with the input or the output stream: they usually just transform, compare or combine values. The operators do work with the streams, in conjunction with these functions.

Project and predicate

How you label a function (eg. “project” or “predicate” function) depends on how you use it.

A function that:

• takes several values
• and returns one new value of any type

may be used as a project function on combining operators (eg. combineLatest or zip).

A function that:

• takes one value
• and returns one new value of any type

may be used as a project function with map.

A function that:

• takes one value
• and returns one new value of type boolean (✔ true or ✘ false)

may be used as a predicate function on filtering operators (eg. filter, takeWhile or skipWhile)

As you may have noticed, a function used as a predicate can also be used as project function on a map operator:

In this example, the function n ⟶ n == 1, that returns ✔ true or ✘ false, is used both as an argument for map as a project and for filter as a predicate. As a result:

• map returns a new stream of boolean values, according to the result of this project function
• filter returns a new stream of filtered values, where the values can pass only if this predicate returns ✔ true

Summary

• Some operators may accept functions as arguments. These functions don’t have to work with the input or the output stream. The operators do.
• How you label a function (eg. “project” or “predicate” function) depends on how you use it. A function is not a project or predicate function in itself.

See also

map vs filter

accumulator