[Haskell-cafe] Idea for a very simple GUI llibrary

Maurí­cio CA mauricio.antunes at gmail.com
Sun Nov 22 19:19:00 EST 2009


Here is a sketch for a library with these properties:

-> Easy to test. All Haskell code can be tested in a text
terminal. Also, testing code that uses the library can also be
done without using a GUI.

-> Extremely easy to document and use.

-> Not even close to Gtk2hs power, but enough for small

-> Could be the first GUI to build on hackage :)

What we need is:

-> MyState. A user suplied type for application state.

-> WidId. A user suplied type for widget identifiers.

-> Gui wi. A type capable of describing an interface with all of
its state. It's an instance of Eq.

-> Event wi. A type for events.

-> Prop. A type for properties than can related to a WidId.

Running an application would be like this:

main = runGUI
         initState  -- An initial MyState.
         event      -- :: MyState -> DiffTime -> Event WidId -> MyState
         props      -- :: WidId -> [Prop]
         action     -- :: MyState -> DiffTime -> IO (Maybe (MyState,Gui WidId))
         timeout    -- :: DiffTime

DiffTime parameters for callbacks are always the time elapsed
since application started.

 From initState and event, the implementation of runGUI can save a
state that optionally changes with time.

 From props, it can get details on what to present in widgets
associated with a WidId (selected state, picture to draw etc.).

action presents a chance for using IO, and optionally change state
and GUI description.

timeout is the maximum time runGUI implementation is allowed to
wait between calls to action.

Examples for those types:

newtype MyState = {
     lastUpdate :: DiffTime,
     builtGui :: Bool,
     earthCoordinates :: (Double,Double),
     map :: SVG,

data WidId = XCoord | YCoord | MapWindow | ReloadButton ...

data Gui widid = TitleWindow (Gui widid)
       | Tabs [(String,Gui widid)]
       | PressButton String widid
       | Selection [String] widid
       | ...
  deriving Eq
       Eq is needed by runGUI to detect if GUI has
       changed after the last call to action.

data Event widid = ButtonPressed widid
       | FileSelected String widid
       | OptionSelected String widid
       | ...

data Prop widid = Active Bool
       | Text String
       | Draw SVG
       | ...

I believe this can represent most kinds of simple applications,
and be efficient enough for practical use.

It's interesting that all of this can be designed, implemented and
tested independent of runGUI implementation. Actually, if you want
a pet project and want to write and design the Haskell part, I may
probably be able to write runGUI for you :)


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