Unsafe head function with descriptive error message.
This function is partial and will throw an error on empty lists. It
should only be used when there's a strong invariant guaranteeing the
list is non-empty.
Why use this instead of a total function?
- Preserves existing type signatures and caller simplicity
- Makes invariant violations fail fast with clear error
messages
- Avoids pushing complexity up the call chain for conditions that
should never occur
- Used specifically in getGenerators where groupBy
never produces empty groups
When to use:
- Internal functions with strong invariants
- Performance-critical code where the invariant is guaranteed
- When converting to total functions would complicate the entire
call chain
When NOT to use:
- Public APIs where callers might pass invalid input
- When the input domain genuinely includes edge cases
- When safety is more important than performance