1. (Originally Unix, now also MS-DOS) A program that processes an input data
stream into an output data stream in some well-defined way, and does no I/O to
anywhere else except possibly on error conditions; one designed to be used as a
stage in a pipeline (see plumbing). Compare sponge.
2. (functional programming) A higher-order function which takes a predicate and
a list and returns those elements of the list for which the predicate is true.
filter p  = 
filter p (x:xs) = if p x then x : rest else rest
rest = filter p xs
See also filter promotion.
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