SRFI 40: A Library of Streams

by Philip L. Bewig

status: withdrawn (2017-08-10)

keywords: Data Structure, Superseded

See also SRFI 41: Streams.


Along with higher-order functions, one of the hallmarks of functional programming is lazy evaluation. A primary manifestation of lazy evaluation is lazy lists, generally called streams by Scheme programmers, where evaluation of a list element is delayed until its value is needed.

The literature on lazy evaluation distinguishes two styles of laziness, called even and odd. Odd style streams are ubiquitous among Scheme programs and can be easily encoded with the Scheme primitives delay and force defined in R5RS. However, the even style delays evaluation in a manner closer to that of traditional lazy languages such as Haskell and avoids an "off by one" error that is symptomatic of the odd style.

This SRFI defines the stream data type in the even style, some essential procedures and syntax that operate on streams, and motivates our choice of the even style. A companion SRFI 41 Stream Library provides additional procedures and syntax which make for more convenient processing of streams and shows several examples of their use.