Why isnt Op class Op op obj origObj impl | op obj origObj -> impl? about fltkhs HOT 6 CLOSED

iiuc, like the technique that supports variadic functions, this technique lets a method have implementations with different arities.

(But this might be wrong, since I'm not quite sure what you mean, without a concrete program, and because I'm not familiar with the Op typeclass either)

from fltkhs.

@sboosali To me that sounds to be too little gain... Different arities can be done by uncurrying

from fltkhs.

Yes, which requires records. However, a design goal of this package is making the many fltk examples (written in C), fltk being extensively documented, as copy-pasteable and "mutually intelligible" as possible.

from fltkhs.

It can be done but it's ugly and it should really be part of the API but here's how to do it. Say you originally had something like :

ui :: IO ()
ui = do
 window <- windowNew ...
 begin window
 button <- buttonNew ...
 ...
 end window
 ...

If you only cared about Window and not any of it's subclasses (like SingleWindow, DoubleWindow etc.) you can do:

within :: Ref Window -> IO () -> IO ()
within window action = do
  begin window
  action
  end window

ui :: IO ()
ui = do
 window <- windowNew ...
 within window $ do
   button <- buttonNew ...
   ...
 ...

However if you want it to work for any Window it becomes ugly:

withinGeneric ::
  (
    Parent win Window,
    Match x ~ FindOp win win (Begin ()),
    Op (Begin ()) x win (IO ()),
    Match y ~ FindOp win win (End ()),
    Op (End ()) y win (IO ())
  )
  => Ref win -> IO () -> IO ()
withinGeneric ref' action = do
  () <- begin ref'
  action
  end ref'

ui :: IO ()
ui = do
 window <- windowNew ...
 withinGeneric window $ do
   button <- buttonNew ...
   ...
 ...

There's a lot of complexity here but in essence the Parent win Window constraint ensures that win is a "subclass" of Window, then the pair of Match x ~ FindOp win win (Begin ()), Op (Begin ()) x win (IO ()) first finds a function called begin on win and then ensure that it's of type IO () and then similarly with end.

In any case this should be provided so feel free to leave this issue open and I'll add it to the API on the next release. You're also free to put up a PR if you want. :)

Edit: You'll also want to add:

{-# LANGUAGE FlexibleContexts, TypeFamilies #-}

from fltkhs.

And to answer your original question about FunDeps, I did try them originally but they didn't work because impl is a function signature like (a -> b -> IO ()). So for example, when setting the font size of a Widget, impl is (Font -> IO ()) which has to be a constraint which is ignored for the purposes of FunDeps.

from fltkhs.

this is exactly what I do (and it work)
it is incredibly ugly though. I agree that it should be added into the library.
I will work on the PR :)

from fltkhs.