docs.md

Rule always_declare_return_types

Group: style
State:
Since: Dart SDK: >= 2.0.0 • (Linter v0.1.4)

has-fix

Description

DO declare method return types.

When declaring a method or function always specify a return type. Declaring return types for functions helps improve your codebase by allowing the analyzer to more adequately check your code for errors that could occur during runtime.

BAD:

main() { }

_bar() => _Foo();

class _Foo {
  _foo() => 42;
}

GOOD:

void main() { }

_Foo _bar() => _Foo();

class _Foo {
  int _foo() => 42;
}

typedef predicate = bool Function(Object o);

index.md

Linter for Dart

Lint Rules

Using the Linter

Supported Lint Rules

This list is auto-generated from our sources.

Rules are organized into familiar rule groups.

• errors - Possible coding errors.

• style - Matters of style, largely derived from the official Dart Style Guide.

• pub - Pub-related rules.

In addition, rules can be further distinguished by maturity. Unqualified rules are considered stable, while others may be marked experimental to indicate that they are under review. Lints that are marked as deprecated should not be used and are subject to removal in future Linter releases.

Rules can be selectively enabled in the analyzer using analysis options or through an analysis options file.

• An auto-generated list enabling all options is provided here.

As some lints may contradict each other, only a subset of these will be enabled in practice, but this list should provide a convenient jumping-off point.

Many lints are included in various predefined rulesets:

• core for official "core" Dart team lint rules.
• recommended for additional lint rules "recommended" by the Dart team.
• flutter for rules recommended for Flutter projects (flutter create enables these by default).

Rules included in these rulesets are badged in the documentation below.

These rules are under active development. Feedback is welcome!

Error Rules

Rule	Description
always_use_package_imports	Avoid relative imports for files in lib/. has-fix
avoid_dynamic_calls	Avoid method calls or property accesses on a "dynamic" target.
avoid_empty_else	Avoid empty else statements. core recommended flutter has-fix
avoid_print	Avoid print calls in production code. flutter has-fix
avoid_relative_lib_imports	Avoid relative imports for files in lib/. core recommended flutter has-fix
avoid_returning_null_for_future	Avoid returning null for Future. has-fix
avoid_slow_async_io	Avoid slow async dart:io methods.
avoid_type_to_string	Avoid .toString() in production code since results may be minified.
avoid_types_as_parameter_names	Avoid types as parameter names. core recommended flutter has-fix
avoid_web_libraries_in_flutter	Avoid using web-only libraries outside Flutter web plugin packages. flutter
cancel_subscriptions	Cancel instances of dart.async.StreamSubscription.
close_sinks	Close instances of dart.core.Sink.
collection_methods_unrelated_type	Invocation of various collection methods with arguments of unrelated types.
comment_references	Only reference in scope identifiers in doc comments.
control_flow_in_finally	Avoid control flow in finally blocks. recommended flutter
diagnostic_describe_all_properties	DO reference all public properties in debug methods. has-fix
discarded_futures	Don't invoke asynchronous functions in non-async blocks. has-fix
empty_statements	Avoid empty statements. recommended flutter has-fix
hash_and_equals	Always override hashCode if overriding ==. core recommended flutter has-fix
invalid_case_patterns	Use case expressions that are valid in Dart 3.0. has-fix
invariant_booleans	Conditions should not unconditionally evaluate to true or to false.
iterable_contains_unrelated_type	Invocation of Iterable.contains with references of unrelated types. core recommended flutter
list_remove_unrelated_type	Invocation of remove with references of unrelated types. core recommended flutter
literal_only_boolean_expressions	Boolean expression composed only with literals.
no_adjacent_strings_in_list	Don't use adjacent strings in list.
no_duplicate_case_values	Don't use more than one case with same value. core recommended flutter has-fix
no_logic_in_create_state	Don't put any logic in createState. flutter
prefer_relative_imports	Prefer relative imports for files in lib/. has-fix
prefer_void_to_null	Don't use the Null type, unless you are positive that you don't want void. recommended flutter has-fix
test_types_in_equals	Test type arguments in operator ==(Object other).
throw_in_finally	Avoid throw in finally block.
unnecessary_statements	Avoid using unnecessary statements.
unrelated_type_equality_checks	Equality operator == invocation with references of unrelated types. core recommended flutter
unsafe_html	Avoid unsafe HTML APIs.
use_build_context_synchronously	Do not use BuildContexts across async gaps. flutter
use_key_in_widget_constructors	Use key in widget constructors. flutter has-fix
valid_regexps	Use valid regular expression syntax. core recommended flutter

Style Rules

Rule	Description
always_declare_return_types	Declare method return types. has-fix
always_put_control_body_on_new_line	Separate the control structure expression from its statement. has-fix
always_put_required_named_parameters_first	Put required named parameters first. has-fix
always_require_non_null_named_parameters	Specify @required on named parameters without defaults. recommended flutter has-fix
always_specify_types	Specify type annotations. has-fix
annotate_overrides	Annotate overridden members. recommended flutter has-fix
avoid_annotating_with_dynamic	Avoid annotating with dynamic when not required. has-fix
avoid_as	Avoid using as.
avoid_bool_literals_in_conditional_expressions	Avoid bool literals in conditional expressions.
avoid_catches_without_on_clauses	Avoid catches without on clauses.
avoid_catching_errors	Don't explicitly catch Error or types that implement it.
avoid_classes_with_only_static_members	Avoid defining a class that contains only static members.
avoid_double_and_int_checks	Avoid double and int checks.
avoid_equals_and_hash_code_on_mutable_classes	Avoid overloading operator == and hashCode on classes not marked @immutable.
avoid_escaping_inner_quotes	Avoid escaping inner quotes by converting surrounding quotes. has-fix
avoid_field_initializers_in_const_classes	Avoid field initializers in const classes.
avoid_final_parameters	Avoid final for parameter declarations.
avoid_function_literals_in_foreach_calls	Avoid using forEach with a function literal. recommended flutter has-fix
avoid_implementing_value_types	Don't implement classes that override ==.
avoid_init_to_null	Don't explicitly initialize variables to null. recommended flutter has-fix
avoid_js_rounded_ints	Avoid JavaScript rounded ints.
avoid_multiple_declarations_per_line	Don't declare multiple variables on a single line.
avoid_null_checks_in_equality_operators	Don't check for null in custom == operators. recommended flutter has-fix
avoid_positional_boolean_parameters	Avoid positional boolean parameters.
avoid_private_typedef_functions	Avoid private typedef functions. has-fix
avoid_redundant_argument_values	Avoid redundant argument values. has-fix
avoid_renaming_method_parameters	Don't rename parameters of overridden methods. recommended flutter has-fix
avoid_return_types_on_setters	Avoid return types on setters. recommended flutter has-fix
avoid_returning_null	Avoid returning null from members whose return type is bool, double, int, or num.
avoid_returning_null_for_void	Avoid returning null for void. recommended flutter has-fix
avoid_returning_this	Avoid returning this from methods just to enable a fluent interface.
avoid_setters_without_getters	Avoid setters without getters.
avoid_shadowing_type_parameters	Avoid shadowing type parameters. core recommended flutter
avoid_single_cascade_in_expression_statements	Avoid single cascade in expression statements. recommended flutter has-fix
avoid_types_on_closure_parameters	Avoid annotating types for function expression parameters. has-fix
avoid_unnecessary_containers	Avoid unnecessary containers. flutter has-fix
avoid_unused_constructor_parameters	Avoid defining unused parameters in constructors. has-fix
avoid_void_async	Avoid async functions that return void. has-fix
await_only_futures	Await only futures. core recommended flutter has-fix
camel_case_extensions	Name extensions using UpperCamelCase. core recommended flutter
camel_case_types	Name types using UpperCamelCase. core recommended flutter
cascade_invocations	Cascade consecutive method invocations on the same reference. has-fix
cast_nullable_to_non_nullable	Don't cast a nullable value to a non nullable type.
combinators_ordering	Sort combinator names alphabetically. has-fix
conditional_uri_does_not_exist	Missing conditional import.
constant_identifier_names	Prefer using lowerCamelCase for constant names. recommended flutter
curly_braces_in_flow_control_structures	DO use curly braces for all flow control structures. core recommended flutter has-fix
dangling_library_doc_comments	Attach library doc comments to library directives. has-fix
deprecated_consistency	Missing deprecated annotation.
directives_ordering	Adhere to Effective Dart Guide directives sorting conventions. has-fix
do_not_use_environment	Do not use environment declared variables.
empty_catches	Avoid empty catch blocks. core recommended flutter has-fix
empty_constructor_bodies	Use ; instead of {} for empty constructor bodies. recommended flutter has-fix
enable_null_safety	Do use sound null safety.
eol_at_end_of_file	Put a single newline at end of file. has-fix
exhaustive_cases	Define case clauses for all constants in enum-like classes. recommended flutter has-fix
file_names	Name source files using lowercase_with_underscores. core recommended flutter
flutter_style_todos	Use Flutter TODO format: // TODO(username): message, https://URL-to-issue.
implementation_imports	Don't import implementation files from another package. recommended flutter
implicit_call_tearoffs	Explicitly tear-off call methods when using an object as a Function. has-fix
join_return_with_assignment	Join return statement with assignment when possible.
leading_newlines_in_multiline_strings	Start multiline strings with a newline. has-fix
library_annotations	Attach library annotations to library directives. has-fix
library_names	Name libraries using lowercase_with_underscores. recommended flutter
library_prefixes	Use lowercase_with_underscores when specifying a library prefix. recommended flutter
library_private_types_in_public_api	Avoid using private types in public APIs. recommended flutter
lines_longer_than_80_chars	Avoid lines longer than 80 characters.
missing_whitespace_between_adjacent_strings	Missing whitespace between adjacent strings.
no_default_cases	No default cases.
no_leading_underscores_for_library_prefixes	Avoid leading underscores for library prefixes. recommended flutter has-fix
no_leading_underscores_for_local_identifiers	Avoid leading underscores for local identifiers. recommended flutter has-fix
no_runtimeType_toString	Avoid calling toString() on runtimeType.
non_constant_identifier_names	Name non-constant identifiers using lowerCamelCase. core recommended flutter has-fix
noop_primitive_operations	Noop primitive operations. has-fix
null_check_on_nullable_type_parameter	Don't use null check on a potentially nullable type parameter. core recommended flutter has-fix
null_closures	Do not pass null as an argument where a closure is expected. recommended flutter has-fix
omit_local_variable_types	Omit type annotations for local variables. has-fix
one_member_abstracts	Avoid defining a one-member abstract class when a simple function will do.
only_throw_errors	Only throw instances of classes extending either Exception or Error.
overridden_fields	Don't override fields. recommended flutter
package_api_docs	Provide doc comments for all public APIs.
package_prefixed_library_names	Prefix library names with the package name and a dot-separated path. core recommended flutter
parameter_assignments	Don't reassign references to parameters of functions or methods.
prefer_adjacent_string_concatenation	Use adjacent strings to concatenate string literals. recommended flutter has-fix
prefer_asserts_in_initializer_lists	Prefer putting asserts in initializer lists.
prefer_asserts_with_message	Prefer asserts with message.
prefer_bool_in_asserts	Prefer using a boolean as the assert condition.
prefer_collection_literals	Use collection literals when possible. recommended flutter has-fix
prefer_conditional_assignment	Prefer using ??= over testing for null. recommended flutter has-fix
prefer_const_constructors	Prefer const with constant constructors. flutter has-fix
prefer_const_constructors_in_immutables	Prefer declaring const constructors on @immutable classes. flutter has-fix
prefer_const_declarations	Prefer const over final for declarations. flutter has-fix
prefer_const_literals_to_create_immutables	Prefer const literals as parameters of constructors on @immutable classes. flutter has-fix
prefer_constructors_over_static_methods	Prefer defining constructors instead of static methods to create instances.
prefer_contains	Use contains for List and String instances. recommended flutter has-fix
prefer_double_quotes	Prefer double quotes where they won't require escape sequences. has-fix
prefer_equal_for_default_values	Use = to separate a named parameter from its default value. recommended flutter
prefer_expression_function_bodies	Use => for short members whose body is a single return statement. has-fix
prefer_final_fields	Private field could be final. recommended flutter has-fix
prefer_final_in_for_each	Prefer final in for-each loop variable if reference is not reassigned. has-fix
prefer_final_locals	Prefer final for variable declarations if they are not reassigned. has-fix
prefer_final_parameters	Prefer final for parameter declarations if they are not reassigned. has-fix
prefer_for_elements_to_map_fromIterable	Prefer 'for' elements when building maps from iterables. recommended flutter has-fix
prefer_foreach	Use forEach to only apply a function to all the elements.
prefer_function_declarations_over_variables	Use a function declaration to bind a function to a name. recommended flutter has-fix
prefer_generic_function_type_aliases	Prefer generic function type aliases. core recommended flutter has-fix
prefer_if_elements_to_conditional_expressions	Prefer if elements to conditional expressions where possible. has-fix
prefer_if_null_operators	Prefer using if null operators. recommended flutter has-fix
prefer_initializing_formals	Use initializing formals when possible. recommended flutter has-fix
prefer_inlined_adds	Inline list item declarations where possible. recommended flutter has-fix
prefer_int_literals	Prefer int literals over double literals. has-fix
prefer_interpolation_to_compose_strings	Use interpolation to compose strings and values. recommended flutter has-fix
prefer_is_empty	Use isEmpty for Iterables and Maps. core recommended flutter has-fix
prefer_is_not_empty	Use isNotEmpty for Iterables and Maps. core recommended flutter has-fix
prefer_is_not_operator	Prefer is! operator. recommended flutter has-fix
prefer_iterable_whereType	Prefer to use whereType on iterable. core recommended flutter has-fix
prefer_mixin	Prefer using mixins.
prefer_null_aware_method_calls	Prefer null aware method calls.
prefer_null_aware_operators	Prefer using null aware operators. recommended flutter has-fix
prefer_single_quotes	Only use double quotes for strings containing single quotes. has-fix
prefer_spread_collections	Use spread collections when possible. recommended flutter has-fix
prefer_typing_uninitialized_variables	Prefer typing uninitialized variables and fields. core recommended flutter has-fix
provide_deprecation_message	Provide a deprecation message, via @Deprecated("message"). core recommended flutter
public_member_api_docs	Document all public members.
recursive_getters	Property getter recursively returns itself. recommended flutter
require_trailing_commas	Use trailing commas for all function calls and declarations. has-fix
sized_box_for_whitespace	SizedBox for whitespace. flutter has-fix
sized_box_shrink_expand	Use SizedBox shrink and expand named constructors.
slash_for_doc_comments	Prefer using /// for doc comments. recommended flutter has-fix
sort_child_properties_last	Sort child properties last in widget instance creations. flutter has-fix
sort_constructors_first	Sort constructor declarations before other members. has-fix
sort_unnamed_constructors_first	Sort unnamed constructor declarations first. has-fix
super_goes_last	Place the super call last in a constructor initialization list.
tighten_type_of_initializing_formals	Tighten type of initializing formal.
type_annotate_public_apis	Type annotate public APIs. has-fix
type_init_formals	Don't type annotate initializing formals. recommended flutter has-fix
unawaited_futures	Future results in async function bodies must be awaited or marked unawaited using dart:async. has-fix
unnecessary_await_in_return	Unnecessary await keyword in return.
unnecessary_brace_in_string_interps	Avoid using braces in interpolation when not needed. recommended flutter has-fix
unnecessary_breaks	Don't use explicit breaks when a break is implied. has-fix
unnecessary_const	Avoid const keyword. recommended flutter has-fix
unnecessary_constructor_name	Unnecessary .new constructor name. recommended flutter has-fix
unnecessary_final	Don't use final for local variables. has-fix
unnecessary_getters_setters	Avoid wrapping fields in getters and setters just to be "safe". recommended flutter has-fix
unnecessary_lambdas	Don't create a lambda when a tear-off will do. has-fix
unnecessary_late	Don't specify the late modifier when it is not needed. recommended flutter has-fix
unnecessary_library_directive	Avoid library directives unless they have documentation comments or annotations. has-fix
unnecessary_new	Unnecessary new keyword. recommended flutter has-fix
unnecessary_null_aware_assignments	Avoid null in null-aware assignment. recommended flutter has-fix
unnecessary_null_aware_operator_on_extension_on_nullable	Unnecessary null aware operator on extension on a nullable type.
unnecessary_null_checks	Unnecessary null checks.
unnecessary_null_in_if_null_operators	Avoid using null in if null operators. recommended flutter has-fix
unnecessary_nullable_for_final_variable_declarations	Use a non-nullable type for a final variable initialized with a non-nullable value. recommended flutter has-fix
unnecessary_overrides	Don't override a method to do a super method invocation with the same parameters. core recommended flutter has-fix
unnecessary_parenthesis	Unnecessary parentheses can be removed. has-fix
unnecessary_raw_strings	Unnecessary raw string. has-fix
unnecessary_string_escapes	Remove unnecessary backslashes in strings. recommended flutter has-fix
unnecessary_string_interpolations	Unnecessary string interpolation. recommended flutter has-fix
unnecessary_this	Don't access members with this unless avoiding shadowing. recommended flutter has-fix
unnecessary_to_list_in_spreads	Unnecessary toList() in spreads.
unreachable_from_main	Unreachable top-level members in executable libraries.
use_colored_box	Use ColoredBox.
use_decorated_box	Use DecoratedBox. has-fix
use_enums	Use enums rather than classes that behave like enums. has-fix
use_full_hex_values_for_flutter_colors	Prefer an 8-digit hexadecimal integer(0xFFFFFFFF) to instantiate Color. flutter has-fix
use_function_type_syntax_for_parameters	Use generic function type syntax for parameters. recommended flutter has-fix
use_if_null_to_convert_nulls_to_bools	Use if-null operators to convert nulls to bools.
use_is_even_rather_than_modulo	Prefer intValue.isOdd/isEven instead of checking the result of % 2.
use_late_for_private_fields_and_variables	Use late for private members with a non-nullable type.
use_named_constants	Use predefined named constants.
use_raw_strings	Use raw string to avoid escapes. has-fix
use_rethrow_when_possible	Use rethrow to rethrow a caught exception. recommended flutter has-fix
use_setters_to_change_properties	Use a setter for operations that conceptually change a property.
use_string_buffers	Use string buffers to compose strings.
use_string_in_part_of_directives	Use string in part of directives.
use_super_parameters	Use super-initializer parameters where possible. has-fix
use_test_throws_matchers	Use throwsA matcher instead of fail().
use_to_and_as_if_applicable	Start the name of the method with to/_to or as/_as if applicable.
void_checks	Don't assign to void. core recommended flutter

Pub Rules

Rule	Description
depend_on_referenced_packages	Depend on referenced packages. core recommended flutter
package_names	Use lowercase_with_underscores for package names. recommended flutter
secure_pubspec_urls	Use secure urls in pubspec.yaml.
sort_pub_dependencies	Sort pub dependencies alphabetically.