Example: List Helpers

List-specific helpers complement the generic map, filter, folds, and Sequence operations. They use u64 positions and return Option when indexing or slicing can fail.

Safe access

( list_get 1 [10, 20, 30]
, list_get 3 [10, 20, 30]
, list_slice 1 3 [10, 20, 30]
, list_slice 3 1 [10, 20, 30]
)

list_get uses a zero-based index. list_slice uses a half-open range and returns None when either bound is invalid or the end precedes the start.

Constructing and reshaping lists

( list_reverse [1, 2, 3]
, list_concat [[1, 2], [], [3, 4]]
, list_repeat 3 "rex"
)

list_concat removes exactly one layer of nesting. list_repeat 0 value returns an empty list.

Searching

let even = \x -> x % 2 == 0 in
( list_any even [1, 3, 4, 5]
, list_all even [2, 4, 6]
, list_find even [1, 3, 4, 6]
, list_find_index even [1, 3, 4, 6]
, list_count even [1, 2, 3, 4, 5, 6]
)

The any, all, and find helpers inspect elements from left to right and stop when their answer is known. The find helpers return the first match. On an empty list, list_any is false, list_all is true, and both find helpers return None.

Partitioning

list_partition (\x -> x < 0) [3, negate 1, 0, negate 4, 2]

The first output list contains matching elements and the second contains rejected elements. Both retain their relative order from the input.