[Haskell-cafe] Instance match surprise

Wojtek Narczyński wojtek at power.com.pl
Mon Feb 1 22:33:46 UTC 2016

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 

> 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?


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