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→Characteristics: This article was flagged as "too technical", so I provided an opening paragraph with a simplified description of traits. |
I've started documenting some known limitations of traits. Sadly, the references to this are largely my communications with the trait researchers and years of personal experience. |
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* ''alias'': an operation that creates a new trait by adding a new name for an existing method
* ''exclusion'': an operation that forms a new trait by removing a method from an existing trait. (Combining this with the alias operation yields a ''shallow rename'' operation).
Note that if you exclude a method from a trait, that method is required to be provided by the class that consumes the trait, or by a parent class that class inherits from. This is because the trait methods themselves might call the excluded method.
Traits are composed in the following ways:
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* Conflicting methods are excluded from the composition.
* Nested traits are equivalent to flattened traits; the composition hierarchy does not affect the traits behaviour. For example, given trait ''S'' = ''A'' + ''X'', where ''X'' = ''B'' + ''C'', then trait ''T'' = ''A'' + ''B'' + ''C'' is the same as ''S''.<ref name="schaerli-ecoop-2003"/>
* A trait may require the consuming class to provide methods for the trait's use. For example, if the trait provides a <code>toJson</code> method, it might require the class to provide a <code>toDict</code> method that the <code>toJson</code> calls to convert to JSON.
==Limitations==
While traits offer significant advantages over many alternatives, they do have their own limitations.
=== Required Methods ===
If a trait requires the consuming class to provide certain methods, the trait cannot know if those methods are [[Semantic_equivalence|semantically equivalent]] to the trait's needs. Worse, for some dynamic languages, such as Perl, the required method can only be identified by a method name, not a full [[Type signature|method signature]], making it harder to guarantee that the required method is appropriate.
=== Excluding Methods ===
If you exclude a method from a trait, that method becomes a 'required' method for the trait because the trait's other methods might call it. If the programming language uses [[Static dispatch|static dispatch]] ("early binding"), method names are bound are [[Compile time|compile time]] and the trait methods might call the trait's implementation of the method and not the corresponding method provided by the class. Conversely, if the programming language uses [[Dynamic dispatch|dynamic dispatch]] ("late binding"), method names are determined at run time and the trait methods might call the class's implementation of the method and not the method provided by the trait. If a method is excluded, the programmer must take this into account.
==Supported languages==
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