[Haskell-cafe] ATs vs FDs
andrewcoppin at btinternet.com
Sat Aug 14 06:19:23 EDT 2010
As I understand it, ATs were invented because FDs are "evil" and must
never be used ever for any purpose. However, it doesn't seem to be
possible to use ATs to do the same things that FDs can do.
You can use ATs to write type functions, which take one type and return
another type. This allows you to express type relationships in a very
elegant way. However, what it does /not/ seem to allow you to do is
express one-to-one relationships.
For example, I'd like to be able to say that the next vector up from a
Vector3 is a Vector4, and the next vector down is a Vector2. And I can
say that. What I can't say is that the *only* next vector up is a
Vector4. And thus, all my code is littered with ambiguous type warnings
because although /currently/ there's only one class instance, somebody
could come along some day and write another one.
What am I missing?
More information about the Haskell-Cafe