On Oct 11, 2012, at 5:30 PM, Mikhail Glushenkov <the.dead.shall.r...@gmail.com>
wrote:
> Hello Simon,
>
> On Thu, Oct 11, 2012 at 11:24 AM, Simon Marlow <marlo...@gmail.com> 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]
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