avoid_untyped_safe_cast

v1.0.0 Warning fpdart

This rule flags a call to Either.safeCast, Option.safeCast or Either.safeCastStrict whose target type was inferred as dynamic because no type arguments were written and the context supplied none.

Why use this rule

safeCast takes its input as dynamic and decides the outcome with value is R. When the call carries no type arguments and nothing in the surrounding context constrains inference, R lands on dynamic — and every value satisfies is dynamic.

The cast can therefore never fail. Either.safeCast always returns Right, Option.safeCast always returns Some, and a validator written to reject malformed input silently accepts everything. Nothing looks wrong at the call site, the analyzer says nothing, and the bad payload surfaces much later as a cast error on some unrelated field.

fpdart’s own API documentation carries this warning on both constructors — “Make sure to specify the types of Either … otherwise this will always return Right!”. This rule enforces it.

See also: fpdart: Either.safeCast, fpdart: Option.safeCast

Don’t

// R infers as dynamic — this is always Right, whatever `json` holds.
final result = Either.safeCast(json, (v) => 'not a map');

Do

Write the target type on the call:

final result = Either<String, Map<String, dynamic>>.safeCast(
  json,
  (v) => 'not a map',
);

Letting the context supply it works just as well, and is not reported:

Either<String, int> parse(dynamic json) =>
    Either.safeCast(json, (v) => 'not an int');

Known limitations

The rule reports on the inferred type, not on the absence of type arguments. A call that omits them but sits in a context that constrains the type — a typed variable, a return position, a typed argument — is correct and stays silent.

That is the whole point: final Either<String, int> r = Either.safeCast(...) behaves exactly as intended, and flagging it would push authors toward redundant annotations.

Configuration

This rule is in the core preset, so it is on with preset: core, preset: recommended or preset: opinionated.

To turn it off:

many_lints.yaml

rules:
  avoid_untyped_safe_cast: false

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

Related rules

• prefer_safe_collection_access — list.first throws where list.head returns None.
• 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.