default roles

[Simon Peyton-Jones]

Wait, that sounds like it induces bad semantics.

I?m not sure this is as bad as you say.

First, in this case Set?s argument has nominal role, so the GND you give would be rejected anyway.  So Set?s sanctity stays unscathed.

But beyond that, when you say
newtype Bar = Bar Int deriving (Eq,Foo)
the ?deriving Foo? bit says

?please implement all the Foo operations on Bar values in precisely the same way that you implement Foo operations on Int.?

Now that might be a stupid thing to say.  There are many ways in which it could be semantically wrong.  One of them is that Ints are ordered one way and Bars are ordered another.  But there may be other ways too.  Perhaps Bars have properties that Ints don?t which mean that inserting should work differently.  A type system cannot hope to catch all of these.

Simon

From: Edward Kmett [mailto:ekmett at gmail.com]
Sent: 11 October 2013 03:09
To: Richard Eisenberg
Cc: David Menendez; glasgow-haskell-users at haskell.org Mailing List; Simon Peyton-Jones
Subject: Re: default roles

Wait, that sounds like it induces bad semantics.

Can't we use that as yet another way to attack the sanctity of Set?

class Ord a => Foo a where
  badInsert :: a -> Set a -> Set a

instance Foo Int where
  badInsert = insert

newtype Bar = Bar Int deriving (Eq,Foo)

instance Ord Bar where
  compare (Bar x) (Bar y) = compare y x

Now you can badInsert into a Set.

If that is still in play then even with all the roles machinery then GND doesn't pass the restrictions of "SafeHaskell". =(

-Edward

On Thu, Oct 10, 2013 at 9:52 PM, Richard Eisenberg <eir at cis.upenn.edu<mailto:eir at cis.upenn.edu>> wrote:

On Oct 10, 2013, at 1:14 PM, David Menendez wrote:
> Sure, but if op uses show internally, we get Int's show, not Age's, right? That seems correct, in that it's doing what GND is supposed to do, but I'll bet it will surprise people.
Yes, you're right. If a method in a subclass uses a superclass method, it uses the base type's instance's method, not the newtype's. Very weird, but I guess it makes sense in its own way. This does show how GND can create instance incoherence even without storing dictionaries in datatypes.

Richard

_______________________________________________
Glasgow-haskell-users mailing list
Glasgow-haskell-users at haskell.org<mailto:Glasgow-haskell-users at haskell.org>
http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/glasgow-haskell-users/attachments/20131011/56ef09ce/attachment-0001.html>