Type Class¶
A type class, as found in Haskell and similar languages, is a named set of functions that a type must implement in order to belong to that class. Unlike an object-oriented interface, a type class is not implemented by the type itself: instances are declared separately, so a type class can be attached to a type after the fact, including to types the author does not own, such as built-in numbers or third-party types.
Type classes provide ad-hoc polymorphism: the same function name, such as == or show, resolves to a different implementation depending on the type of its argument, chosen by the compiler at compile time rather than looked up in a vtable at runtime.
PHP has no type class mechanism. Ad-hoc polymorphism over an existing type can only be approximated with interfaces implemented by the type’s own class, free functions with manual type checks, or match expressions branching on gettype(). There is no way to retroactively attach a shared contract to a type PHP’s engine already knows about, such as int or a class from a third-party library, without wrapping it.
Related : Interface, Polymorphism, Generics, Overloading, Type System
