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

[Magicloud Magiclouds]

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
>



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