Take a list slice

A slice items[start:stop:step] returns a new list inside a Result. Start is included and stop is excluded. Negative bounds count from the end, omitted bounds run to the ends, a negative step walks backwards, and bounds outside the list clamp to its ends:

def middle(items: &list[i64]) -> Result[list[i64], AllocError]:
    items[1:-1]

def main() -> Result[(), AllocError | IoError]:
    items = [10, 20, 30, 40]?
    print(middle(&items)?)?
    print(items[-100:100]?)?
    print(items[3:1]?)?
    print(items[::-2]?)?
    print(items)?
    Ok(())

[20, 30]
[10, 20, 30, 40]
[]
[40, 20]
[10, 20, 30, 40]

Bounds must be i64, and the step is a NonZero[i64]. Even an empty result can fail to allocate its list header; use ? or match to handle AllocError.

The slice copies each selected element with copy(), so nested lists are duplicated and the source keeps its elements. Element types that cannot be copied, such as std.bytes.Bytes, are rejected. For those, call items.slice(start, stop) on an owned temporary, such as a list returned by a function: it transfers the selected elements and drops the rest. Use copy(items)?.slice(start, stop) only when the elements can be copied anyway. If allocation fails, the temporary is cleaned up in full; a borrowed source remains unchanged.