Re: unsafeCoerce and type aliases under type applications

On 12/04/2011 12:40 PM, Felipe Almeida Lessa wrote:

On Sun, Dec 4, 2011 at 3:30 PM, Erik Hesselink wrote:

...

It's not just type families though, right? I mean, a (Map A) cannot be coerced to a (Map B), even if B is a newtype over A, since they might have different Ord instances, and thus a different map structure.

It depends on what you call "safe". The bug Brent Yorgey was referring to allows you to get sefaults and the like. The bug you're describing violates some invariants, but these invariants are not expressed in the type system and won't make a hard crash of your program.

This problem could be addressed by the hypothetical safe-coercion operator requiring any relevant constructors to be in scope. (Alas, then it would have to be magical syntax.) -Isaac