Overlapping and incoherent and intentionally omitted instances

Iavor Diatchki iavor.diatchki at gmail.com
Sun Aug 10 20:12:04 UTC 2014


Such a pragma sounds useful, and is very much like the "fails" instance
from the "Instance chains" paper.  You may also be interested in ticket
#9334 (https://ghc.haskell.org/trac/ghc/ticket/9334), which proposes an
alternative to overlapping instances, and I just updated it to point to


On Sun, Aug 10, 2014 at 7:19 AM, Henning Thielemann <
schlepptop at henning-thielemann.de> wrote:

> Am 29.07.2014 um 12:02 schrieb Johan Tibell:
>  P.S. For e.g. INLINABLE we require that you mention the function name
>> next to the pragma (which means that you can e.g. put the pragma after
>> the declaration). What's the rationale to not require
>> {-# OVERLAPPING Show [Char] #-}
>> here? Perhaps it's too annoying to have to repeat the types?
> Once I proposed a pragma for documenting intentionally unimplemented
> instances. In this case there is no instance you can write a pragma in
> front of. Your OVERLAPPING syntax would be conform with the one of
> https://ghc.haskell.org/trac/ghc/ticket/7775
> Maybe NOINSTANCE can be reconsidered in the course of the introduction of
> the OVERLAP pragma?
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