[Haskell-cafe] Problem with existential type and instance.

[Roman Cheplyaka]

What do you mean? MonadIO m is a constraint. You have to supply some type that
satisfies this constraint. IO is one such type; you could use any other.

Alternatively, if you *don't* want to store a MonadIO dictionary in the Nothing
case, create your own Maybe type:

  data MaybeMonadIO
    = NoMonadIO
    | forall m . MonadIO m => JustMonadIO (m ())

  newtype EventHandlers = EventHandlers { onDeleteWindow :: MaybeMonadIO }

* Magicloud Magiclouds <magicloud.magiclouds at gmail.com> [2014-07-28 15:50:05+0800]
> I see. Thank you.
> 
> Any anyway I could use (MonadIO m) here, in Nothing data type?
> 
> 
> On Mon, Jul 28, 2014 at 3:47 PM, Roman Cheplyaka <roma at ro-che.info> wrote:
> 
> > * Magicloud Magiclouds <magicloud.magiclouds at gmail.com> [2014-07-28
> > 13:46:53+0800]
> > > Hi,
> > >
> > >   For code like below, how to make it compilable?
> > >
> > > data EventHandlers = forall m. MonadIO m => EventHandlers {
> > onDeleteWindow
> > > :: Maybe (m ()) }
> > > instance Default EventHandlers where
> > >   def = EventHandlers Nothing
> >
> > GHC has to store some MonadIO dictionary in the existential type, but it
> > cannot
> > figure out which dictionary to store (it can be any).
> >
> > You can tell it which one to use by supplying a type annotation, e.g.:
> >
> >   def = EventHandlers (Nothing :: Maybe (IO ()))
> >
> > Roman
> >
> 
> 
> 
> -- 
> 竹密岂妨流水过
> 山高哪阻野云飞
> 
> And for G+, please use magiclouds#gmail.com.
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