[Haskell-cafe] Re: Can we come out of a monad?

[Kevin Jardine]

So far the comments here only increase my confusion (which as I say,
is not bad because it means that I am learning something!).

As a Haskell newbie, the first thing I learned about monads is that
they have a type signature that creates a kind of mud you can't wash
off.

eg.

f :: String -> MyMonad String

By mentioning the monad, you get to use its special functions but as a
hard price, you must return a value with a type signature that locks
it within the monad (although you can remove the signature within
other monads using "<-").

As some people have hinted, perhaps the problem is that most Haskell
newbies are introduced to monads through the IO monad and other monads
are different.

When I plunged into Haskell earlier this year, I had no problem with
understanding static typing, higher level functions and even
separating pure functions from IO functions.

The more I learn about monads, however, the less I understand them.
I've seen plenty of comments suggesting that monads are easy to
understand, but for me they are not.

Cheers,
Kevin

On Jul 30, 12:29 pm, Tillmann Rendel <ren... at informatik.uni-
marburg.de> wrote:
> C K Kashyap wrote:
> > I am of the
> > understanding that once you into a monad, you cant get out of it?
>
> That's not correct.
>
> There are many monads, including Maybe, [], IO, ... All of these monads
> provide operations (>>=), return and fail, and do notation implemented
> in terms of these functions, as a common interface. Using just this
> common interface, you cannot "get out of the monad".
>
> But most if not all monads also provide additional operations, specific
> to the monad in question. Often, these operations can be used to "get
> out of that monad". For example, with Maybe, you can use pattern matching:
>
>    case do x <- return 5
>            fail "some message"
>            return (x + 3) of
>      Just a   ->  a
>      Nothing  ->  0
>
> So we can get out of many monads, but we need to know which one it is to
> use the appropriate operation.
>
> Kevin Jardine wrote:
> > I'm still trying to understand how monads interact with types so I am
> > interested in this as well.
>
>  From my point of view, the most important fact about monads is:
>
>    There is nothing special about monads!
>
> The type class Monad behaves like very other type class. A monadic type
> constructor behaves like every other type constructor. The type class
> methods (>>=), return and fail behave like every other type class
> method. There is nothing special about monads.
>
> The only speciality of monads is do notation, but do notation is only a
> syntactic convenience, and can be translated into calls of (>>=), return
> and fail, which, as noted above, are not special in any way.
>
> So, back to your question, since there is nothing special about monads,
> monads do not interact with types in any special way.
>
>    Tillmann
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