RE: The FunctorM library

On 23 March 2005 21:28, Iavor Diatchki wrote:

Hello,

On Wed, 23 Mar 2005 15:51:04 +0000, Ross Paterson wrote:

...

On Wed, Mar 23, 2005 at 09:49:43AM -0000, Simon Marlow wrote:

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Does anyone else have any comments on the suggestions from Iavor and Thomas in this thread? I'm happy to make changes, but only if there's a concensus. The proposals so far seems to be

1) add dist method 2a) make Functor a superclass of FunctorM 2b) make Functor a *sub*class of FunctorM 2c) make Functor a subclass of Monad 2d) make Functor a superclass of Monad

Just to avoid confusion I think the suggestions were: class Functor f => Monad f where ... class Functor f => FunctorM f where ...

I know the first one differs from the Haskell report, but perhaps this is a flaw in the library design that should be fixed. The practical concern is compatability with existing programs, but if a program breaks at all it should be very easy to fix --- at most one line per monad instance, and a Haskell implementation will be able to pinpoint the missing instances.

This means making a pretty big step: we would be deliberately removing support for some valid Haskell 98 programs. That's a bit radical IMHO. We could make Monad.Monad (the Haskell 98 version) a different class from Control.Monad.Monad to keep support for Haskell 98 code. This would break some code too - but maybe that's not such a bad thing, we want people to move away from the Haskell 98 non-hierarchical libraries anyway. In fact, I think (for GHC users at least) we should be advocating writing new code using the option -hide-package haskell98 which removes the Haskell 98 libraries from the namespace. Cheers, Simon