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
filledin, and typechecked 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 righthand 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 nondefault 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 righthand side of the default
type signature for bar
must be something that is alphaequivalent 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).