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

[Ömer Sinan Ağacan]

Hi Daniel,

This won't work. Let's say there has been two keypresses and I called
handleKey two times. First time it updates the `dat` and returns it
but how can I pass that updated `dat` object to the same function in
second invocation?

---
Ömer Sinan Ağacan
http://osa1.net


2014-03-02 23:38 GMT+02:00 Daniel Trstenjak <daniel.trstenjak at gmail.com>:
>
> Hi Ömer,
>
>> In a sense I'm like trying to emulate something like closures with
>> mutable closed-over data in impure languages.
>
> One way is to keep the specific data completely out of the Widget and
> use a closure to hold the data:
>
>     data Widget = Widget
>        { handleKey :: Key -> Widget
>        , draw      :: Int -> Int -> Int -> Int -> IO ()
>        , getFocus  :: Widget
>        , loseFocus :: Widget
>        }
>
>     someWidget :: SomeWidgetData -> Widget
>     someWidget dat = Widget
>        { handleKey = \key -> case key of 'A' -> someWidget $ dat { ... } ; ...
>        , draw      = \x y w h -> ...
>        , getFocus  = someWidget $ dat { focus = True }
>        , loseFocus = someWidget $ dat { focus = False }
>        }
>
>
> Greetings,
> Daniel
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