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

Oops, wrong mail account for my last email. Apologies. What I'm trying to accomplish is being able to write haskell libraries for the interpreter that don't use the interpreter's predefined Value types, without having to edit the Value type itself and add a new constructor (i.e. it's abstracted away). I believe the concept is called 'userdata' in Lua, for example. In C interpreters it is usually accomplished by throwing away type information and casting to 'void*' and then casting back to the type to get it out again. That said, I think I may defer this until I understand the ins and outs of Haskell's type system a little better. I think a parametrized type will be the only way to do it. The only reason I thought GADTs may be able to do this is because I read some literature that suggested GADTs could be used as a kind of typecase construct (I think it was 'Fun with Phantom Types'?) but I could have very easily misunderstood it. 2009/7/13 Chris Eidhof :

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:

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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 :

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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:

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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