On 01.02.2016 21:34, Ben wrote:
But there *could* be other instances the compiler just can't see at the moment (because it's in another module that's not imported). Someone else could then import this module and that other module and observe inconsistent instances.
Okay, I get it. In another module there might be instances Eq and Typeable for ANY, and then the instances would indeed overlap. It makes sense.
To avoid that problem, the type class system *never* commits to "negative" information; if an instance choice is only valid because there *isn't* a possible instance in scope then that instance choice is not valid after all.
I will have to produce many instances by hand. Couldn't the compiler just postpone the its overlap checking until linking? Or might I forbid creation of Eq and Typeable instances of ANY? -- Wojtek