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/