[Haskell-cafe] Monad layering and DSL

[Tom Ellis]

On Sun, Jan 25, 2015 at 10:39:48PM +0100, Gautier DI FOLCO wrote:
> 2015-01-25 9:37 GMT+01:00 Tom Ellis <
> tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk>:
> 
> > On Sat, Jan 24, 2015 at 07:46:52PM +0100, Gautier DI FOLCO wrote:
> > > data ImapF next =
> > >          Select DirectoryName (Maybe DirectoryDescription -> next)
> > >        | Noop (DirectoryDescription -> next)
> > >        | Search MailSearch (Maybe [UID] -> next)
> > > -- Functor instance
> > > type Imap = Free ImapF
> > >
> > > searchAll :: Imap (Maybe [UID])
> > > searchAll = liftF $ Search undefined id
> > > select :: DirectoryName -> Imap (Maybe DirectoryDescription)
> > > select directory = liftF $ Select directory id
> > >
> > > My main problem is the following: if I do a select of an unknown
> > directory,
> > > I should stop the computation.
> >
> > What do you mean you "should stop the computation"?  Do you mean the
> > interpretation of the free monad should be forced to stop?  If so, doesn't
> > making the type of the `Select` constructor
> >
> >     Select DirectoryName (DirectoryDescription -> next)
> >
> > achieve this?  That way if the interpreter talks issues a select command to
> > the server and fails to receive a `DirectoryDescription` in return then it
> > is forced to stop.
> 
> If I do it this way, the evaluation is forced to be stopped, I have no way
> to react to this error.

Could you explain more precisely what you are trying to achieve, perhaps
with an example?

Tom