[hoc-devel] Implicit variables for instance variables

[Andre P. <oz...@al...>]

(Hooray, first post to hoc-devel! :)

One idea which I think would be really nice: use implicit parameters to 
pass instance variables to methods.  So, normally you'd write something 
like this (taken from the ExpressionParser sample I just checked into 
CVS):

     obj #. var = obj # getIVar var

     -- ep_evaluate :: NSButton a
     --             -> EPController a
     --             -> IO ()
     ep_evaluate sender self = do
       expressionTextField <- self #. _expressionTextField
       expression <- expressionTextField # stringValue >>= haskellString
       ...

This is quite cumbersome, because you need to write extra statements 
just to get the instance variable from the object.  Using implicit 
variables, you could instead write:

     -- ep_evaluate :: ( ?expressionTextField :: NSTextField a )
     --             => NSButton a
     --             -> EPController a
     --             -> IO ()
     ep_evaluate sender self = do
       expression <- expressionTextField # stringValue >>= haskellString

This saves writing the #. helper function, and also saves having to 
write a statement that uses #. for each instance variable you want to 
pull out in every method.  The only problem I can see with this is that 
the Template Haskell syntax has to be extended to include implicit 
variables -- do you know how to do this?  If not, I'll have to do some 
hacking myself or ask Ian Lynagh about it.

Note that if you don't _use_ the implicit variables in the function, 
that's perfectly OK too, which is the behaviour we'd want.

     {-# OPTIONS -fglasgow-exts #-}

     module ImplicitParameterTest where

     aMethod :: ( ?instanceVariable1 :: String
                , ?instanceVariable2 :: Int )
             => IO ()
     aMethod = do
       putStrLn "hello!"

^ The above compiles perfectly.

(In Mocha, I was using the "unsafePerformIO $ newIORef ..." hack to do 
this because it's just so much more convenient than an explicit, 
monadic getIVar function, but implicit variables achieves the same goal 
and would be safer.)


-- 
% Andre Pang : trust.in.love.to.save