# [Haskell-cafe] Join and it's relation to >>= and return

Ron de Bruijn rondebruijn at yahoo.com
Wed Jun 9 11:20:08 EDT 2004

```--- "Iavor S. Diatchki" <diatchki at cse.ogi.edu> wrote:
> hi ron,
>
> here are the relations between the two formulations
>
> map f  m     = m >>= (return . f)
> join m         = m >>= id
>
> m >>= f      = join (fmap f m)
>
> there are quite a few general concepts that you need
> to understand in what sense monads are monoids, but
> to understand how monads work you don't need to know
> that.
>
> Ron de Bruijn wrote:
>
> >I am pretty sure, that >>= is to monads what * is
> to
> >for example natural numbers, but I don't know what
> the
> >inverse of >>= is. And I can't really find it
> anywhere
> >on the web(papers, websites, not a single sole does
> >mention it.
> >
> >
> this is not quie correct.  (join & return) for a
> (*,1) or (+,0) for the set of integers.  however
> those operations on
> integers have more structure than join and return.
>
> there is no operation for "inverse". in mathematical
> terms:
> monads are a monoid (given that the notion is
> generalized considerably
> from its usual use), and not a group.
> if one was to add such an operation
> (i am not sure what it would do), but it would be of
> type:
> inverse :: M a -> M a (and of course must satisfy
> some laws)
>
> also while you are pondering these things, it may be
> useful to use
> the "backward join" (=<<) :: (a -> m b) -> (m a -> m
> b).
> the reason for that is that strictly speaking tha
> arrow in the middle
> is different from the left and the right arrows
>
> i am not sure how useful this information is for
> you,
> but if you have more questions ask away
> -iavor
I found out what a group is:
A group is a monoid each of whose elements is
invertible.

Only I still find it weird that join is called a
multiplication, because according to the definition of
multiplication, there should be an inverse. I think,
thus that multiplication is only defined on a group.
And now still remains: why do they call it a
multiplication, while by definition it's not. Or
should I understand it as: there's a concept called
multiplication and for different structures there's a
definition? I think, now I think over it, that it
would seem logical.

It could be possible that the definition is incorrect,
though. Does anyone knows of a definition that is more
general (and not absolute nonsens ;))?

The information you give me is *very* usefull, because
I don't just want to work with monads, I truly want to
understand them.

Ron

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