101 lines
2.5 KiB
Plaintext
101 lines
2.5 KiB
Plaintext
https://dart.dev/language/pattern-types#constant[Constant patterns] should not be used with type literals.
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== Why is this an issue?
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Using constant patterns with type literals is most likely a mistake.
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For example, the following code
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[source,dart]
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----
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bool isANumber(Object? o) {
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if (o case num) { // Checks if `o` is `num`, not if it is a `num`
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return true;
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}
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return false;
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}
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----
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will always return `false` for any input *value* of type `num` (as in `isANumber(42)`), and will returns `true` only when the input is the *type* `num` (as in `isANumber(int)`).
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This is because a constant pattern compares the value of the provided constant against the value being matched, and not against its type.
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The original intent of checking whether an `Object? o` is a `num` or not, should be expressed via a https://dart.dev/language/pattern-types#variable[typed variable pattern]:
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[source,dart]
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----
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bool isANumber(Object? o) {
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if (o case num n) { // Checks if `o` is a `num` and assigns the cast value to n
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return true;
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}
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return false;
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}
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----
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If the intent of the code is to only check whether the input is a `num` or not, then the `n` variable is not necessary, and a value discard can be used:
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[source,dart]
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----
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bool isANumber(Object? o) {
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if (o case num _) { // Checks if `o` is a `num`, and discards the cast value
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return true;
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}
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return false;
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}
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----
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== How to fix it
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Convert the https://dart.dev/language/pattern-types#constant[constant pattern] to a https://dart.dev/language/pattern-types#variable[typed variable pattern] by adding a ``++_++`` (value discard) to the type.
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=== Code examples
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==== Noncompliant code example
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[source,dart,diff-id=1,diff-type=noncompliant]
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----
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bool isANumber(Object? o) {
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if (o case num) {
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return true;
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}
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return false;
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}
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----
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==== Compliant solution
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[source,dart,diff-id=1,diff-type=compliant]
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----
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bool isANumber(Object? o) {
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if (o case num _) {
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return true;
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}
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return false;
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}
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----
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== Resources
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* Dart Docs - https://dart.dev/tools/linter-rules/type_literal_in_constant_pattern[Dart Linter rule - type_literal_in_constant_pattern]
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* Dart Docs - https://dart.dev/language/pattern-types[Language - Pattern types]
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ifdef::env-github,rspecator-view[]
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'''
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== Implementation Specification
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(visible only on this page)
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=== Message
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Use 'TypeName _' instead of a type literal.
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=== Highlighting
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The type literal: e.g. `num` in `if (o1 case num) { }`.
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'''
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== Comments And Links
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(visible only on this page)
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endif::env-github,rspecator-view[]
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