Orphan instance

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An orphan instance is a type class instance for class C and type T which is neither defined in the module where C is defined nor in the module where T is defined.

Type class instances are special in that they don't have a name and cannot be imported explicitly. This also means that they cannot be excluded explicitly. All instances defined in a module A are imported automatically when importing A, or importing any module that imports A, directly or indirectly.

Say you want to define an alternative instance to an existing instance. This is a bad thing, since if two instances for the same class/type pair are in scope, then you cannot describe in Haskell 98 which instance to use. If you want to use multiple instances for the same class/type, you have to ensure that they are never imported together in a module somewhen. It is almost impossible to assert that, or put differently, it would reduce the composability of libraries considerably.

The Monad instance of Either is a good example. It is not defined where Either is defined, thus all of its Monad instances must be orphan. Instead it is defined both in Control.Monad.Error of the Monad Transformer Library and in Control.Monad.Trans.Error of its lightweight cousin the 'transformers' package. Since some packages use MTL and others 'transformers' it becomes difficult to use that instance at all, although both instances are equivalent! Practical advice: The explicit-exception package with its Exceptional might be a better choice to use since it avoids the current problem with orphan Monad instances of Either.

Actually, non-orphan instances can avoid definition of multiple instances. For defining an instance you have to import the class and the type and then you will automatically have the according non-orphan instances imported, too. If you want to define a new instance then the compiler will reject it immediately.

When Orphan Instances can be useful

It is worth noting that Orphan Instances can be viewed as a mechanism for writing modules of code with a fixed typed interface, but parameterized over the choice of implementation. In this case, Orphan Instances act as a sort of plugin architecture for providing alternative implementations with a uniform interface.

A basic treatment of the relationship between type classes and modules (in the SML sense of modules) can be found at http://www.mpi-sws.org/~dreyer/papers/mtc/main-short.pdf and http://www.cse.unsw.edu.au/~chak/papers/modules-classes.pdf

See also