Lazy evaluation

Evaluation delayed until a result is demanded, with sharing of the delayed computation's result. A lazy list can expose a constructor before computing its tail; this permits productive infinite structures and lazy feedback streams. A non-strict language can implement its semantics this way without making every function non-strict.

Represent a cursor by an underlying consuming iterator plus an optional cached head. Without a cache, its abstract sequence is the iterator's sequence; with cached $v$, it is $[v]$ concatenated with the remaining iterator sequence. current fetches and caches exactly one value when needed, repeated reads reuse it, and advance discards it. Availability testing need not fetch a value, and construction need not query the input. This avoids forcing a future stream element that the client may never request.