status: final (2008/1/24)See also SRFI 40: A Library of Streams.
Streams, sometimes called lazy lists, are a sequential data structure containing elements computed only on demand. A stream is either null or is a pair with a stream in its cdr. Since elements of a stream are computed only when accessed, streams can be infinite. Once computed, the value of a stream element is cached in case it is needed again.
Streams without memoization were first described by Peter Landin in 1965. Memoization became accepted as an essential feature of streams about a decade later. Today, streams are the signature data type of functional programming languages such as Haskell.
This Scheme Request for Implementation describes two libraries for operating on streams: a canonical set of stream primitives and a set of procedures and syntax derived from those primitives that permits convenient expression of stream operations. They rely on facilities provided by R6RS, including libraries, records, and error reporting. To load both stream libraries, say: