array_utils design

array_utils collects small generic helpers on Array[T] in src/array_utils.mbt. This page explains the choices behind them and the behaviour that maintainers must keep stable.

Constraints from luna-generic

Each function asks for the weakest constraint it needs. reverse and reverse_inplace need none, find and the uniformity checks need Eq, and arr_max and arr_min need Compare. arr_sum and zero_arr need only AddMonoid from luna-generic, that is zero() and +, so they work for every type with an additive monoid and not only for numbers. arr_abs_sum needs Num for negation and Compare to find the sign.

The package imports luna-generic under the alias lg, and src/alias.mbt makes AddMonoid and Num usable without the prefix inside the package. Public signatures name them @luna-generic.AddMonoid and @luna-generic.Num.

Functions instead of methods

MoonBit does not let a package define methods on Array, which belongs to the core library, so the helpers are package-level functions.

Empty arrays

The functions that need an element to start from read arr[0] and therefore panic on an empty array: arr_max, arr_min, same and map_same. They do not return an Option. The other functions have a natural answer for an empty array and return it: zero() for the sums, true for same_to and map_same_to, None for find, and an empty array for reverse. Keep this split when adding functions, or document the exception.

Allocation

reverse, zero_arr, map_same and map_same_to allocate a new array. reverse_inplace mutates its argument, and the other functions only read.

Maintenance notes

• Update this page and the API reference whenever a function is added or removed, or its constraints or edge-case behaviour change, and regenerate src/pkg.generated.mbti with moon info.
• The doc comment of reverse_inplace in the source says that it panics on an empty array. It does not: the loop does not run, and the array is left unchanged.