[Haskell-cafe] Monad layering and DSL

[Gautier DI FOLCO]

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.
>
> Tom
>
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Hello,

Thanks for your answer.
If I do it this way, the evaluation is forced to be stopped, I have no way
to react to this error.
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