On Oct 11, 2012, at 5:30 PM, Mikhail Glushenkov wrote:

Hello Simon,

On Thu, Oct 11, 2012 at 11:24 AM, Simon Marlow wrote:

...

On 08/10/2012 20:11, Mikhail Glushenkov wrote:

...

I couldn't find anything on the interplay between orphan instances and Safe Haskell both in the Haskell'12 paper and online. Is this something that the authors of Safe Haskell are aware of/are intending to fix?

[...] I don't know what we should do about this. Disallowing orphan instances seems a bit heavy-handed. David, Simon, any thoughts?

What about detecting duplicate instances at link time? We could put information about all instances defined in a given module into the .comment section of the corresponding .o file and then have a pre-link step script extract this information from all .o files in the program and check that there are no duplicate or conflicting instances.

You have a bigger problem coming. Some extensions make multiple instances OK, even in Safe Haskell. For example: {-# LANGUAGE FlexibleInstances, IncoherentInstances, Safe #-} module Over where data Nil = Nil newtype Cons a = Cons a class Number n where value :: n -> Integer instance Number Nil where value Nil = 0 instance Number a => Number (Cons a) where value (Cons n) = value n + 1 instance Number (Cons (Cons Nil)) where value (Cons (Cons Nil)) = 2012 naturals = nats Nil where nats :: Number n => n -> [Integer] nats n = value n : nats (Cons n) Here we have two different instances Number (Con (Cons Nil)) at play, because it gives you: *Over> value (Cons (Cons Nil)) 2012 *Over> take 5 naturals [0,1,2,3,4]

On Oct 11, 2012, at 6:23 PM, Mikhail Glushenkov wrote:

Hello,

On Thu, Oct 11, 2012 at 3:54 PM, MigMit wrote:

...

You have a bigger problem coming. Some extensions make multiple instances OK, even in Safe Haskell. For example:

{-# LANGUAGE FlexibleInstances, IncoherentInstances, Safe #-} [...]

Safe Haskell already disallows OverlappingInstances.

It doesn't. Overlapping instances are allowed in safe modules, provided that they won't be used in other modules.

I hate to admit it, but it seems to me now that one would need dependent types to guarantee Set's good behavior (so that the dictionary for the Ord instance is contained within the Set type). Отправлено с iPhone Oct 11, 2012, в 18:42, Mikhail Glushenkov написал(а):

Hello,

On Thu, Oct 11, 2012 at 4:33 PM, MigMit wrote:

...

On Oct 11, 2012, at 6:23 PM, Mikhail Glushenkov wrote:

...

On Thu, Oct 11, 2012 at 3:54 PM, MigMit wrote:

...

You have a bigger problem coming. Some extensions make multiple instances OK, even in Safe Haskell. For example:

{-# LANGUAGE FlexibleInstances, IncoherentInstances, Safe #-} [...]

Safe Haskell already disallows OverlappingInstances.

It doesn't. Overlapping instances are allowed in safe modules, provided that they won't be used in other modules.

I stand corrected. Still, I think that something will have to be done about IncoherentInstances if the aforementioned restriction will be added.

-- () ascii ribbon campaign - against html e-mail /\ www.asciiribbon.org - against proprietary attachments