A function that maps streams to streams. If [T → A] is the set of all streams of elements of set A indexed by time T then, for example, a stream transformer that maps a pair of streams into one stream is a function of the form
F : [T → A]2 → [T → A]
Stream transformers are often defined in the equivalent but logically simpler form
G : [T → A]n × T → A
where G(a,t) = F(a)(t) for stream a and time t, i.e. G(a,t) is the element on the output stream F(a) at time t.