[Haskell-cafe] name for monad-like structure?
wren ng thornton
wren at freegeek.org
Tue Apr 28 20:08:02 EDT 2009
Michael Vanier wrote:
> Luke Palmer wrote:
> > Michael Vanier wrote:
> > > Are you sure it supports
> > > (>>) :: m a -> m b -> m b
> > >
> > > and not
> > > mplus :: m a -> m a -> m a ?
> > Yeah, you're right. It's basically a monad where the type a is
> > fixed to be (), so you just have
> > (>>) :: m () -> m () -> m ()
> > That's a monoid.
> Got it. Thanks.
If the return is important ---and the structure can be parameterized by
non-() types, and the binop is actually mplus not (>>)--- then it may
also be a monoid generated from a semigroup. This is the case when
there's an (M a) which can't be generated by return and which serves as
the identity for the binop. Prime examples are monoids similar to Maybe
which are generated by Nothing, return, and any semigroup on the
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