```python
LADDER = (("m", 1000), ("cm", 10), ("mm", 1))


def split_length(mm: int) -> str:
    if not isinstance(mm, int):
        raise ValueError("Length must be a whole number.")
    if mm < 0:
        raise ValueError("Length cannot be negative.")

    result = []
    for unit, factor in LADDER:
        count = mm // factor
        if count > 0:
            result.append(f"{count}{unit}")
        mm %= factor

    return " ".join(result) or "0mm"
```