[Haskell-cafe] Haskell extension/improvement

[Ron de Bruijn]

I was playing with QuickCheck and I just wanted to put
all of my tests in a list, but that's not possible,
because of the fact that a list can only contain
values of the same type. 

So concrete I would like to write down this:

prop_RevRev xs = reverse (reverse xs) == xs
  where types = xs::[Int]

--trivial test
prop_Simple = True == True 

whatIwant::(Testable a)=>[a]
whatIwant = [prop_RevRev, prop_Simple]

testAll::IO ()
testAll = do sequence_ $ map quickCheck whatIwant

quickCheck can work on all Testable values, so why
can't a list (any datastructure) can't hold Testable
values?

What this gives you is, a lot more flexibility. Just
like you can define your functions on interfaces, now
you can use interfaces in your datastructures.  

I can't think of any reason why it couldn't be
implemented in any Haskell compiler. 

I saw this page: 
http://www.haskell.org/hawiki/ExistentialTypes

but you still have to explicitly add the relation via
Obj (and that's bad). 

Regards, Ron

P.S. If it already exist in this exact way, how is it
called?






		
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