[Haskell-cafe] Parameterisations of Monads

Luke Palmer lrpalmer at gmail.com
Tue Feb 5 10:38:08 EST 2008

On Feb 5, 2008 7:48 AM, Matthew Sackman <matthew at wellquite.org> wrote:
> So I was thinking how dull and uninspiring the current definiton of
> Monad really is and came up with some more interesting
> parameterisations. The only problem with this one is I'm a) not sure if
> it still is a Monad and b) very unsure if it's of any use. There's the
> possibility that chucking Cont in there or using newtype to simultate
> multiple arrows / type lambdas may lead to more interesting instances,
> but can anyone think of exciting use cases for this stuff?
> Feel free to fill in the instances! It's also not a parameterisation
> I've seen before.

I can't!  That's because all the instances except for (->) have free
type variables in a covariant position, so I'd be forced to used
undefined all over the place.  And the State instance just confuses
me... :-)

However, I think most Arrows would work as the first parameter, it's
just not clear they would be useful.


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