Re: overlapping instances and constraints

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? -- Ben