[Haskell-cafe] free vs. operational vs. free-operational

[oleg at okmij.org]

> By the way, I'm also interested in knowing if extensible-effects have some
> relation to MonadPlus or Applicative in any way.

The Eff monad is a genuine monad, and all monads are Applicatives. So,
extensible-effects directly relates to Applicative.

As Oliver Charles noted, the extensible-effects package provides the Choose
effect, with the operation
        choose :: Member Choose r => [a] -> Eff r a
to non-deterministically select an element from a list. It is easy to
see that Choose is as alias of MonadPlus: given choose one can
implement mplus and mzero (the latter is choose []). Conversely,
MonadPlus lets us implement Choose. The signature of choose says the
resulting computation will have the Choose effect -- and, since the
effect label |r| is left polymorphic, the computation may have an
unspecified number of other effects (including IO). 

> > handler. So, we get another implementation of LogicT, this time in
> > terms of extensible effects.
>
> This sounds very interesting - is this work available for us to look at
> anywhere?

Actually yes, from the very slightly updated
        http://okmij.org/ftp/Haskell/extensible/Eff.hs
Please search for 'Soft-cut'. The implementation is so trivial that it
didn't seem worth mentioning before. The most interesting part is
        reflect :: VE a r -> Eff r a
which is the inverse of admin (aka, reify). It is gratifying that
monadic reification-reflection can be implemented with extensible-effects
_generically_, once and for all.