[Haskell-cafe] Safe Haskell and instance coherence

[Mikhail Glushenkov]

Hello,

On Thu, Oct 11, 2012 at 3:54 PM, MigMit <miguelimo38 at yandex.ru> 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. If we add a
requirement that it must be unambiguous which instance declaration a
given type class constraint resolves to (taking into account instances
defined in all modules), I think it will be necessary to also disallow
IncoherentInstances.

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