If only it were that easy. Sadly, it's not. Let's look at the following example: data Test = Test { foo :: Int, bar :: Char, baz :: Bool } smallPrint t = concatMap (\f -> show $ f t) [foo, bar, baz] In this code the list [foo, bar, baz] should have the type [exists a. Show a => Test -> a]. If we explicitly specify the type, replacing the exists with a forall, then GHC complains about not being able to match Int, Char and Bool against type a. Forall is not the same as exists and GHC only implements the former. ________________________________________ From: Bulat Ziganshin [bulat.ziganshin@gmail.com] Sent: 19 April 2009 22:07 To: Niemeijer, R.A. Cc: haskell-cafe@haskell.org Subject: Re[2]: [Haskell-cafe] ANNOUNCE: Utrecht Haskell Compiler (UHC) -- first release Hello R.A., Sunday, April 19, 2009, 11:46:53 PM, you wrote:
Does anybody know if there are any plans to incorporate some of these extensions into GHC - specifically the existential typing ?
it is already here, but you should use "forall" keyword instead odf "exists" -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com