Re: [Haskell-cafe] Haskell extension/improvement

--- Martin_Sj�gren wrote:

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I was playing with QuickCheck and I just wanted to

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all of my tests in a list, but that's not

On Mon, 16 Aug 2004 09:01:45 -0700 (PDT), Ron de Bruijn wrote: put possible,

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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

...

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).

Well, if you write

data Test = forall a. Testable a => Test a

instance Testable Test where property (Test x) = property x

you at least get the "unwrapping" for free, and can write things like

mapM_ quickCheck [Test prop_revrev, Test prop_trivial, Test prop_something]

Regards, Martin

I didn't expect these replies (including one mentioning the HList idea(the enforced ordening is nice, though)), while I tried to be precise. The idea is that you don't write down the Test constructor in any place, because you(that's the compiler) can check that all the values you put in the list(or any other datastructure) belong to a certain class. Everything that can be derived should be derived. Regards, Ron __________________________________ Do you Yahoo!? Take Yahoo! Mail with you! Get it on your mobile phone. http://mobile.yahoo.com/maildemo