The Haskell Ports Library (HPL)

Ports are an abstraction for modelling variables whose values evolve over time
without the need to resort to mutable variable, such as IORefs. More precisely,
a port represents all values that a time-dependent variable successively takes
as a stream, where each element of the stream corresponds to a state change -
we can also say that a port represents a time series. Moreover, a port supports
concurrent construction of the time series, or stream of values.

WWW: http://www.cse.unsw.edu.au/~chak/haskell/ports/

-- Oliver Braun
obraun@FreeBSD.org