[Haskell-cafe] I'm trying to design a GUI library -- a design question

[Niklas Haas]

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On Mon, 3 Mar 2014 11:14:41 +0100, Daniel Trstenjak <daniel.trstenjak at gmail.com> wrote:
> 
> Hi Ömer,
> 
> I don't think that you need the 'HasWidgets' class, it really doesn't give you a lot.
> Something like a function 'withFocused' would be a lot more useful:
> 
>    withFocused :: (Widget -> Widget) -> State Program ()
> 
> 
> Greetings,
> Daniel
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This looks like a use case for a lens to me.

> focused :: Lens' Program Widget
> over focused :: (Widget -> Widget) -> Program -> Program

> (focused %=) :: MonadState Program m => (Widget -> Widget) -> m ()
> zoom focused :: (MonadState Widget m, MonadState Program n) => m a -> n a