oleg@okmij.org wrote:
Andrew Coppin wrote:
What we're really trying to do here is attach additional information to a value - information which exists only in the type checker's head, but has no effect on runtime behaviour.
Haskell has had the ability to attach arbitrary many pieces of (type-level) data to arbitrary types for a long time -- ever since multi-parameter type classes and functional dependencies have been introduced. Nowadays, type families accomplish the same with fewer keystrokes. One merely needs to define a type family type family Attrib n index :: * which maps the existing type n and the index to an arbitrary type.
Ah, good point. I hadn't thought of that! So it seems you can indeed attach arbitrary attributes to any type, any time. However, I don't immediately see a way to construct a "safe" string type and an "unsafe" string type, so that you can write code which distinguishes between them, but that existing code that just expects a plain ordinary string still works. You could newtype string as two new types and attach whatever attributes you want to these types, but then normal string functions wouldn't work. Any ideas?