Hey Kev, The types are "thrown away" during compile time. Therefore, if you have a constructor "VWrapper :: a -> Value" nothing is known about that "a" when you scrutinize it. What you could do, however, is something like this:
data Value a where VInt :: Integer -> Value Integer ... VWrapper :: a -> Value a
And then you can write a function doSomething:
doSomething :: Value String -> String doSomething (VWrapper s) = s
HTH, -chris On 13 jul 2009, at 12:41, Kev Mahoney wrote:
Hi there,
I'm currently writing an interpreter that I would like to be able to use with other haskell programs. I would like to be able to pass along arbitrary types though the interpreter. I've seen hints that GADTs can do this, but I am having trouble understanding them.
So far, I've learnt you can do this:
data Value where VInt :: Integer -> Value ... VWrapper :: a -> Value
which can let you encode arbitrary 'dynamic' types into Value. I was hoping to be able to pattern match to get the value out again e.g.
doSomething :: Value -> .... doSomething (VWrapper String s) = .....
Also, anything that can help me out with GADTs in general will be much appreciated.
Thanks, Kevin. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe