Global type class instances
In Haskell, type class instances are global and unique. For any type, it can only have one instance of a type class and it is implicitly imported into anything that imports a file with the instance or a file that imports a file with an instance.
For example, we declare a data type
Person with an instance for
In another module, we import
A. This includes the type class instances that
Person has implemented. We cannot create a new instance of
Show Person. The compiler will complain if we try to make a new instance of
In a third module we only import
B, but do not import
A. Again, we cannot create a new instance of
Show Person because it has been implicitly imported because
A belongs to a library package called
person. Now we make a new executable package called
website that depends on
person and we import the
Person type, but we realize we forgot to add an instance of
Eq so we decide to declare it in the website package.
This works, but the compiler will complain that we have declared an orphan instance. Orphan instances can cause problems if we declare an instance for a type in two different files and then a third file imports both of these files. That would bring in two instances of the same typeclass/type pair and that is not permissible in Haskell. We want to avoid orphan instances as much as possible.
There are two common workarounds:
Define all the type class instances where the type is defined. This is easy if you own the code or you are willing to maintain a branch of the library.
Create a newtype of the type we want more type classes for. This will avoid orphan instances, but require a lot more code and may be annoying if there are a lot of type specific functions we want to use.
Are orphan instances ever acceptable?
If you are declaring type instances in an executable package or a test package that is not shared, declaring orphan instances generally is not a problem because nothing else will import those instances and it will not contaminate other libraries.
We can silence the compiler with a simple annotation at the top any file with orphan instances.
If you have any private libraries, it might be ok to declare orphan instances as well.