Result¶

Result models the explicit outcome of an operation. It represents either success (Ok) or failure (Err), without relying on exceptions for control flow.

Result is about flow, not about error representation.

When to use¶

Use Result when failure is part of normal behavior — validation, parsing, rule evaluation, boundary checks. Return Ok(value) for success, Err(error) for failure.

Variants¶

Ok[ValueType] — Wraps a successful value. is_ok is True, is_err is False.
Err[ErrorType] — Wraps an error. is_ok is False, is_err is True.

Both support structural pattern matching through __match_args__.

Common operations¶

is_ok / is_err — Branch on the variant (property, not method)
value — Unwrap the success payload
error — Unwrap the error payload
map(fn) — Transform success, pass errors through
map_error(fn) — Transform error, pass success through
flat_map(fn) — Chain operations that return Result
get_value_or(default) — Fallback on error
get_value_or_else(fn) — Compute fallback from the error

Design¶

Result is a Protocol — Ok and Err are concrete implementations. You construct Ok(...) or Err(...) but type-annotate with Result[ValueType, ErrorType].