[Haskell-cafe] Safe Haskell and instance coherence

Mikhail Glushenkov the.dead.shall.rise at gmail.com
Thu Oct 11 16:23:18 CEST 2012


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

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