[Haskell-beginners] Understanding reason for Monad

[Travis Cardwell]

Hi Pietro,

The reply I wrote yesterday was sent when I ran out of time, but I have
since realized that I failed to get to the point.  Sorry about that!

The demo shows that you can use `hashPassword` with `IO` and
`ByteString`, as if the type was the following:

    hashPassword :: Int -> ByteString -> IO ByteString

The use of type classes allows the function to be used with other types
as well, however.  This makes the function more flexible for different
people's needs.

For example, a project may use this function in a user registration
request handler.  Perhaps that request handler needs to write to a
database as well as perform logging.  It can be implemented using a
monad that has the context required for these features, so that the it
can do all of these things without being concerned with configuration
details.  The monad may have the following constraints:

* `MonadRandom m` so that `hashPassword` can be used
* `MonadLogger m` so that it can log in the configured way
* `MonadDatabase m` (using some application-defined monad) so that it
  has access to the application database

If `hashPassword` had the above `IO` type, then it would not work in
such an application monad directly.  If there is a `MonadIO` instance,
then one could use `liftIO`, but some application monads specifically
do *not* have `MonadIO` instances to make it more difficult to simply
run things in `IO`.

I am out of time again, but I hope this two-part reply is helpful.

Cheers,

Travis