This is fairly the same as
http://hackage.haskell.org/trac/ghc/ticket/8021
but it's rewritten a little
Interfaces
Interface is a generalization of the class.
Now class become just *instances* of interface
For backward compatibility compiler should calculate interface by itself,
but it should allow to write interfaces to everybody (more detail at "Maybe
Problems").
/-- it is calculated by the Compiler or is written directly/
class interface Monoid (a :: k) where /-- like Typeable/
mempty :: *
mappend :: * -> * -> *
class Monoid a where ...
class Monad m => Monoid m where ...
class Arrow c => Monoid c where ...
It is easy to use them with functions:
foo :: (Monad m => Monoid m , Monad m ) => m a
bar :: (Arrow c => Monoid c, Arrow c) => c b a
These classes are open, you could lately add
class MyClass c => Monoid c where ...
We use like Typeable (a :: k) because:
() => Monoid a :: *
Monad m => Monoid m :: * -> *
Arrow c => Monoid c :: * -> * -> *
Backward compatibility
If class have single constraint, you could write without it.
If class have many constraints, you could write without it, if it is empty
one.
bar :: MonadPlus m => m a <<==>> bar :: () => Monad m => MonadPlus m =>
m a
foo :: Monoid a => a <<==>> foo :: () => Monoid a => a
baz :: Monad m => Monoid m => m a <<==>> baz :: () => Monad m => Monoid m
=> m a
Better than superclasses
we already have
class Num a where ...
But we wish to generalize it.
It's easy now, we just add:
class Additive a where
addplus = ...
class Multiplicative a where
multprod = ...
class (Additive a, Multiplicative a) => Num a where
(+) = addplus
(*) = multprod
foo :: (Additive a, Multiplicative a) => Num a => a -> a -> a
Maybe Problems
1)
If we have already class
class Foo a where
data F :: ...
type S :: ...
data family G ...
what interface do we have ?
Possible reply - no data in the interface.
2) What is the interface of Typeable ?
2.1) What is the interface of class with unwritten interface?
Possible reply - the same as class, so it is easy to Compiler.
3)
Misfeature - to allow write interface manually
Why? You can't change it later.
So, it must be deprecated from the beginning.
--
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http://haskell.1045720.n5.nabble.com/Interfaces-the-Golden-Path-of-Haskell-tp5732208.html
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