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 <ch...@eidhof.nl>:
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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