6.8.5. Default method signatures

DefaultSignatures
Since

7.2.1

Allows the definition of default method signatures in class definitions.

Haskell 98 allows you to define a default implementation when declaring a class:

class Enum a where
  enum :: [a]
  enum = []

The type of the enum method is [a], and this is also the type of the default method. You can change the type of the default method by requiring a different context using the extension DefaultSignatures. For instance, if you have written a generic implementation of enumeration in a class GEnum with method genum, you can specify a default method that uses that generic implementation. But your default implementation can only be used if the constraints are satisfied, therefore you need to change the type of the default method

class Enum a where
  enum :: [a]
  default enum :: (Generic a, GEnum (Rep a)) => [a]
  enum = map to genum

We reuse the keyword default to signal that a signature applies to the default method only; when defining instances of the Enum class, the original type [a] of enum still applies. When giving an empty instance, however, the default implementation (map to genum) is filled-in, and type-checked with the type (Generic a, GEnum (Rep a)) => [a].

The type signature for a default method of a type class must take on the same form as the corresponding main method’s type signature. Otherwise, the typechecker will reject that class’s definition. By “take on the same form”, we mean that the default type signature should differ from the main type signature only in their contexts. Therefore, if you have a method bar:

class Foo a where
  bar :: forall b. C => a -> b -> b

Then a default method for bar must take on the form:

default bar :: forall b. C' => a -> b -> b

C is allowed to be different from C', but the right-hand sides of the type signatures must coincide. We require this because when you declare an empty instance for a class that uses DefaultSignatures, GHC implicitly fills in the default implementation like this:

instance Foo Int where
  bar = default_bar @Int

Where @Int utilizes visible type application (Visible type application) to instantiate the b in default bar :: forall b. C' => a -> b -> b. In order for this type application to work, the default type signature for bar must have the same type variable order as the non-default signature! But there is no obligation for C and C' to be the same (see, for instance, the Enum example above, which relies on this).

To further explain this example, the right-hand side of the default type signature for bar must be something that is alpha-equivalent to forall b. a -> b -> b (where a is bound by the class itself, and is thus free in the methods’ type signatures). So this would also be an acceptable default type signature:

default bar :: forall x. C' => a -> x -> x

But not this (since the free variable a is in the wrong place):

default bar :: forall b. C' => b -> a -> b

Nor this, since we can’t match the type variable b with the concrete type Int:

default bar :: C' => a -> Int -> Int

That last one deserves a special mention, however, since a -> Int -> Int is a straightforward instantiation of forall b. a -> b -> b. You can still write such a default type signature, but you now must use type equalities to do so:

default bar :: forall b. (C', b ~ Int) => a -> b -> b

We use default signatures to simplify generic programming in GHC (Generic programming).