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.
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