Re: [Haskell-cafe] Safe Haskell and instance coherence

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]