Re: [Haskell-cafe] Are GADTs what I need?

Then you could add a specific constructor for String. The main point is: the case construct only works for values, not for types. There is no typecase construct. If you want to have certain restrictions on the 'a', such as the Show class, you could also do something like this:

data Value where VWrapper :: (Show a) => a -> Value

If you could elaborate a bit on what you're trying to accomplish (from a higher viewpoint) then maybe we can help you some more. -chris On 13 jul 2009, at 17:42, Kev Mahoney wrote:

Thanks, that helps.

I was hoping to not have to parametrize Value as there is a fair bit of code to change, and it cascades down through the data structures (maybe a forall a . Value a will help here?)

I will have a go at this approach. In case anyone is interested the code is at http://github.com/KMahoney

2009/7/13 Chris Eidhof :

...

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