A predicate function is "a function that either returns TRUE or FALSE." While a predicate functional "takes a vector and a predicate function and do something useful."***
In other words, the predicate functionals take in
.x, which is a vector, a dataframe, or a list, and test the predicate on every element of
.x. For example, you can test if every element is numeric with the
is.numeric() predicate from R-Base, or if the mean of some elements is under 5 with this mapper:
~mean(.x) < 5.
Which of these functions is NOT a predicate?