John Meacham wrote:
every single parameter type class whole parameter is of kind *
class Foo a where
automatically declares a data type defined as
data Foo = exists a . Foo a => Foo_ a
(where Foo_ is some internal, non user accessable name)
and an instance
instance Foo Foo where
method (Foo_ x) = method x
...
I don't much like this, it seems like unnecessary sugar to me. My first
preference would be not to do this at all.
My second preference would be to do this a bit more generally:
every type class
class Foo a b c where
automatically declares a data type defined as
data Foo t = exists a b c . Foo a b c => Foo_ (t a b c)
(where Foo_ is some internal, non user accessable name)
every single parameter type class whole parameter is of kind *
declares an instance
instance Foo (Foo Identity) where
method (Foo_ x) = method x
...
It's still pretty ugly. I don't like special-casing classes that happen
to have a particular kind signature.
--
Ashley Yakeley
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