On Mon, Aug 30, 2010 at 11:24:06PM -0700, strejon wrote:
> I'm aware of phantom types and the like, but I've been unable to
> work out how to use them (or another type system extension)
> to properly track "validity" on the type level. I'd want something
> like:
>
> validate :: Certificate Possibly_Valid -> Maybe (Certificate Valid)
>
> With later functions only accepting values of type "Certificate Valid".
>
> Is there a simple way to do this?
Yup. just do it just like you say :)
declare your certificate like so (note: a is not used in the body)
> data Certificate a = Certificate { .. }
then the valid data type
> data Valid
There is no need for a Possibly_Valid type as it can be represented by
just leaving the type unbound. so you will have
validate :: forall a . Certificate a -> Maybe (Certificate Valid)
so validate will take any certificate, and perhaps return a validated
one. Then just use (Certificate Valid) in the types of functions that
require valid certificates.
This also means your functions are polymorphic in their validity by
default, like
> mapName :: (String -> String) -> Certificate a -> Certificate a
will work on valid or invalid certificates.
Just note that when changing a phantom type you need to reconstruct the
type fully. so for
> data A
> data B
> data Foo a = Foo Int
> conv :: Foo A -> Foo B
you can't write
> conv x = x
you need to write
> conv (Foo x) = Foo x
since the argument is changing type.
John
--
John Meacham - ⑆repetae.net⑆john⑈ - http://notanumber.net/
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