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SRFI-41 Streams are based on two mutually-recursive abstract data types: An object of the stream abstract data type is a promise that, when forced, is either stream-null or is an object of type stream-pair. An object of the stream-pair abstract data type contains a stream-car and a stream-cdr, which must be a stream. The essential feature of streams is the systematic suspensions of the recursive promises between the two data types.

The object stored in the stream-car of a stream-pair is a promise that is forced the first time the stream-car is accessed; its value is cached in case it is needed again. The object may have any type, and different stream elements may have different types. If the stream-car is never accessed, the object stored there is never evaluated. Likewise, the stream-cdr is a promise to return a stream, and is only forced on demand.