fmap = ...
from Monad MyMonad derive Functor MyMonad
With the from_derive_ clause, we are telling exactly from which "<=" declaration to pull the definition from. The part of "from" should have already been written or derived, so we know exactly which instance the user is speaking about.
More refinements to the syntax could be done, for example if we have:
instance Functor a <= Applicative a where
fmap = ..
instance Applicative a <= Monad a where
pure = ...
(<*>) = ...
Then, writing "from Monad MyMonad derive Functor MyMonad" would go through the entire tree of "reverse instance declarations" and create instances for Applicative, and from that a Functor one (of course, this should fail if we have more than one path, then the user should write the path explicitly as "from Monad M derive Applicative M; from Applicative M derive Functor M"). But it has the advantage of allowing later addition of classes in the path, that would be derived when recompiling the code that uses it.
2011/7/25 Ryan Ingram
<ryani.spam@gmail.com>
My guess is that nobody has put forward a clear enough design that solves all the problems. In particular, orphan instances are tricky.
Here's an example:
module Prelude where
class (Functor m, Applicative m) => Monad m where
return :: a -> m a
(>>=) :: m a -> (a -> m b) -> m b
(>>) :: m a -> m b -> m b
a >> b = a >>= const b
pure = return
(<*>) = ap
fmap = liftM
module X where
data X a = ...
module Y where
instance Functor X where fmap = ...
module Z where
instance Monad X where
return = ...
(>>=) = ...
-- default implementation of fmap brought in from Monad definition
module Main where
import X
import Z
foo :: X Int
foo = ...
bar :: X Int
bar = fmap (+1) foo -- which implementation of fmap is used? The one from Y?
-- ryan
On Sun, Jul 24, 2011 at 8:55 PM, Ivan Lazar Miljenovic
<ivan.miljenovic@gmail.com> wrote:
On 25 July 2011 13:50, Sebastien Zany <
sebastien@chaoticresearch.com> wrote:
> I was thinking the reverse. We can already give default implementations of class operations that can be overridden by giving them explicitly when we declare instances, so why shouldn't we be able to give default implementations of operations of more general classes, which could be overridden by a separate instance declaration for these?
>
> Then I could say something like "a monad is also automatically a functor with fmap by default given by..." and if I wanted to give a more efficient fmap for a particular monad I would just instantiate it as a functor explicitly.
you cannot do such overriding.
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