On 2/28/06, Ben Rudiak-Gould
Simon Peyton-Jones wrote:
- A program that type checks can have its meaning changed by adding an instance declaration
- Similarly adding "import M()" can change the meaning of a program (by changing which instances are visible
- Haskell would need to be a lot more specific about exactly where context reduction takes place.
I think all of these problems would go away if overlap was permitted within a module but forbidden across modules. Are there uses of overlapping instances for which this isn't flexible enough?
Certainly! In HSP [1] there is a class (simplified here) class IsXML xml where toXML :: xml -> XML data XML = Element .... | CDATA String that deals with how things should be represented as XML. There are a number of basic instances for this, such as instance IsXML String where toXML = CDATA instance (Show a) => IsXML a where toXML = toXML . show The intention of the latter is to be a default instance unless another instance is specified. These instances can be found in the base HSP module, but the idea is that HSP users should be able to work with their own datatypes and only need to define the translation into XML via instanciating IsXML. This would have to be done in the user modules, so overlap across module boundaries are essential for this to work. :-) /Niklas [1] http://www.cs.chalmers.se/~d00nibro/hsp/