filter

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. In Haskell:

filter p [] = []

filter p (x:xs) = if p x then x : rest else rest

where

rest = filter p xs

See also filter promotion.

[Jargon File]