Monoid

funstruct.typeclasses.monoid

Monoid: a Semigroup with an identity element (empty).

Laws

combine(empty, a) == a (left identity) combine(a, empty) == a (right identity)

Monoid = Semigroup + identity. The identity element lets you: - Start a fold with a "zero" value - Construct an "empty" container (Writer.pure needs this) - Use combine in reduce/fold without requiring non-empty input

When to use (vs Semigroup): Use Monoid when you need a starting point / default / empty. Use Semigroup when you only ever combine existing values.

Business examples

• Writer output: start with empty log, accumulate entries
• Fold over a collection: sum([], start=0), concat([], start="")
• Default config: empty config is the identity for merge
• Counters/metrics: start at 0, combine by addition

Monoid dataclass

Bases: Semigroup

A Semigroup with an identity element (empty).

Source code in funstruct/typeclasses/monoid.py

@final
@dataclass(frozen=True)
class Monoid(Semigroup):
    """A Semigroup with an identity element (empty)."""

    typ: type
    combine: Callable
    empty: object