Enumeration and paired iteration

enumerate(iterable[, start: i64]) and zip(left, right) are intrinsic sources for for loops and comprehensions, and can start local pipelines. Pipelines also have .enumerate(start=0) and .zip(right) methods. They compose the existing loop plans and allocate no iterator object, snapshot, or item buffer. Inputs evaluate once in source order; both zip inputs evaluate even if one is empty. Owned items use inline tuples, allowing one tuple binding or two unpacked bindings. Nested adapters work with owned items.

Enumeration yields (i64, element), starting at zero by default. Its counter increments with checked arithmetic only after another element is available. A last index of i64.MAX is valid; a further item traps at the adapter’s source location. continue advances the source and keeps the count aligned. Enumerating a forward list or view, as enumerate(xs), enumerate(&xs), enumerate(v) or iter(&xs).enumerate(), without a start counts with the loop’s own row position. That count cannot overflow and needs no separate counter. Indexing the same &list[T] parameter or view with it then needs no bounds check.

Zip yields (left element, right element) and stops when either input ends. It tests the left condition before the right, including generator advancement. When the right generator ends first, the left can have one unmatched pending item; cleanup drops it. Generator bodies therefore may have one extra left side effect. break, return, and propagation drop all remaining owned inputs.

Each input follows the ordinary iteration ownership rules. Borrowed elements retain their reference types and source loans. Such adapters require two unpacked bindings and bind the references directly, without constructing a tuple containing references. Aliasing exclusive inputs and invalidating mutation remain errors. An inner borrowed adapter cannot be used as an outer adapter’s tuple-valued input. dictionary.items() yields two references; it can start a pipeline but cannot become a nested tuple containing references.

The calls do not construct first-class iterators, support arbitrary iterator protocols, or implement zip strictness. Pipeline enumeration also accepts the start keyword. Reverse traversal can supply either input. A user function or callable binding named enumerate or zip retains its normal call behavior.

The free forms and methods yield the same items: enumerate(&names, 1) and iter(&names).enumerate(1) are interchangeable as a loop source or pipeline start. Use the method form to enumerate or zip partway through a chain, after a filter or skip, where it counts or pairs only the items that reach it. reversed is not a pipeline adapter: reverse the source instead, as in iter(reversed(&values)) or zip(reversed(&values), &values).