Re: [Haskell-cafe] Why the reluctance to introduce the Functor requirement on Monad?

I'll give my two cents about some design I've been thinking about. Instead of trying to derive all instances automatically, the programmer should explicitly tell them (so the problems about conflicting implementations would be minimised). I attach a piece of code of what I think could be done: instance Functor a <= Monad a where -- notice the reversed "<=" 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

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 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.

I believe this has been proposed before, but a major problem is that you cannot do such overriding.

-- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com IvanMiljenovic.wordpress.com

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