[Haskell-cafe] A use case for *real* existential types

[Leon Smith]

On Fri, May 10, 2013 at 9:00 AM, Andres Löh <andres at well-typed.com> wrote:


> > This twist is very simple to deal with if you have real existential
> types,
> > with the relevant part of the interface looking approximately like
> >
> > init :: exists a. IO (Inotify a)
> > addWatch :: Inotify a -> FilePath -> IO (Watch a)
> > rmWatch :: Inotify a -> Watch a -> IO ()
>
> You can still do the ST-like encoding (after all, the ST typing trick
> is just an encoding of an existential), with init becoming "like
> runST":
>
> > init :: (forall a. Inotify a -> IO b) -> IO b
> > addWatch :: Inotify a -> FilePath -> IO (Watch a)
> > rmWatch :: Inotify a -> Watch a -> IO ()
>

Right, but my interface the Inotify descriptor has an indefinite extent,
 whereas your interface enforces a dynamic extent.   I'm not sure to what
degree this would impact use cases of this particular library,  but in
general moving a client program from the the first interface to the second
can require significant changes to the structure of the program,   whereas
moving a client program from the second interface to the first is trivial.
   So I'd say my interface is more expressive.

Best,
Leon
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/haskell-cafe/attachments/20130510/063e219c/attachment.htm>