6.6.3. Stand-alone deriving declarations¶

StandaloneDeriving¶

Since:: 6.8.1
Status:: Included in GHC2021

Allow the use of stand-alone deriving declarations.

GHC allows stand-alone deriving declarations, enabled by StandaloneDeriving:

data Foo a = Bar a | Baz String

deriving instance Eq a => Eq (Foo a)

The syntax is identical to that of an ordinary instance declaration apart from (a) the keyword deriving, and (b) the absence of the where part.

However, standalone deriving differs from a deriving clause in a number of important ways:

The standalone deriving declaration does not need to be in the same module as the data type declaration. (But be aware of the dangers of orphan instances (Orphan modules and instance declarations).
In most cases, you must supply an explicit context (in the example the context is (Eq a)), exactly as you would in an ordinary instance declaration. (In contrast, in a deriving clause attached to a data type declaration, the context is inferred.)

The exception to this rule is that the context of a standalone deriving declaration can infer its context when a single, extra-wildcards constraint is used as the context, such as in:
```
deriving instance _ => Eq (Foo a)
```
This is essentially the same as if you had written deriving Eq after the declaration for data Foo a. Using this feature requires the use of PartialTypeSignatures (Partial Type Signatures).
Unlike a deriving declaration attached to a data declaration, the instance can be more specific than the data type (assuming you also use FlexibleInstances, Instance termination rules). Consider for example
```
data Foo a = Bar a | Baz String

deriving instance Eq a => Eq (Foo [a])
deriving instance Eq a => Eq (Foo (Maybe a))
```
This will generate a derived instance for (Foo [a]) and (Foo (Maybe a)), but other types such as (Foo (Int,Bool)) will not be an instance of Eq.
Unlike a deriving declaration attached to a data declaration, GHC does not restrict the form of the data type. Instead, GHC simply generates the appropriate boilerplate code for the specified class, and typechecks it. If there is a type error, it is your problem. (GHC will show you the offending code if it has a type error.)

The merit of this is that you can derive instances for GADTs and other exotic data types, providing only that the boilerplate code does indeed typecheck. For example:
```
data T a where
   T1 :: T Int
   T2 :: T Bool

deriving instance Show (T a)
```
In this example, you cannot say ... deriving( Show ) on the data type declaration for T, because T is a GADT, but you can generate the instance declaration using stand-alone deriving.

The down-side is that, if the boilerplate code fails to typecheck, you will get an error message about that code, which you did not write. Whereas, with a deriving clause the side-conditions are necessarily more conservative, but any error message may be more comprehensible.
Under most circumstances, you cannot use standalone deriving to create an instance for a data type whose constructors are not all in scope. This is because the derived instance would generate code that uses the constructors behind the scenes, which would break abstraction.

The one exception to this rule is DeriveAnyClass, since deriving an instance via DeriveAnyClass simply generates an empty instance declaration, which does not require the use of any constructors. See the deriving any class section for more details.

In other ways, however, a standalone deriving obeys the same rules as ordinary deriving:

A deriving instance declaration must obey the same rules concerning form and termination as ordinary instance declarations, controlled by the same flags; see Instance declarations and resolution.
The stand-alone syntax is generalised for newtypes in exactly the same way that ordinary deriving clauses are generalised (Generalised derived instances for newtypes). For example:
```
newtype Foo a = MkFoo (State Int a)

deriving instance MonadState Int Foo
```
GHC always treats the last parameter of the instance (Foo in this example) as the type whose instance is being derived.