prefer_unit_over_void

v1.0.0WarningFixConfigurablefpdart

This rule flags an fpdart type parameterised with void — TaskEither<Failure, void>, Option<void>, and so on.

Why use this rule

void is not a value in Dart. You cannot pass it, store it, or hand it to the next step of a pipeline. A TaskEither<Failure, void> therefore stops composing: flatMap on it has nothing meaningful to bind, and callers end up unwrapping to null and re-deriving “did it succeed” from that — which is exactly the manual result-handling fpdart exists to delete.

Unit is fpdart’s answer. It is a real type with exactly one value, so it says “succeeded, with nothing to report” while staying a value the pipeline can carry.

See also: fpdart: Unit

Don’t

TaskEither<Failure, void> save(User user) => throw UnimplementedError();

Do

TaskEither<Failure, Unit> save(User user) => throw UnimplementedError();

The value to return is the unit constant:

TaskEither<Failure, Unit> save(User user) => TaskEither.tryCatch(
  () async {
    await _db.save(user);
    return unit;
  },
  (error, stackTrace) => Failure.from(error),
);

Quick fix

A quick fix replaces the void type argument with Unit and adds the package:fpdart/fpdart.dart import if it is missing. It can be applied across a whole file at once.

The fix deliberately stops there. Once the type says Unit, a body that returned nothing no longer compiles, and you have to write return unit; — the fix will not guess where in an arbitrary body that belongs. A fix that rewrites control flow to satisfy a type annotation is not one anybody should apply without reading it.

Known limitations

A member marked @override is not reported by default: its signature is fixed by the supertype, so the change belongs there instead. Set ignore_overrides: false to report those too.

many_lints:
  rules:
    prefer_unit_over_void:
      ignore_overrides: false

rules:
  prefer_unit_over_void:
    ignore_overrides: false

Option	Type	Default	Description
`ignore_overrides`	bool	`true`	Skip members marked `@override`, whose signature is fixed by the supertype

Configuration

This rule is in the opinionated preset. With a lower preset, enable it by name with prefer_unit_over_void: true.

To turn it off:

rules:
  prefer_unit_over_void: false

To keep the rule on but skip certain paths, use per-rule exclude.

avoid_ad_hoc_left_type — A pipeline only composes when every step shares one error type.
avoid_bare_await_in_do — Awaiting a raw Future inside a Do block escapes the block’s tracking.
avoid_dollar_outside_do_frame — Calling a Do block’s extraction function from a nested callback unwinds through code that cannot handle it.
avoid_either_of_future — A Future nested in Either or Option escapes the error channel.