OVERLAPPABLE/OVERLAPPING/OVERLAPS pragmas are confusing

[Johan Tibell]

The proposed change to my library is here:
https://github.com/tibbe/cassava/pull/95/files

We remove the OverlappingInstances pragma and instead add an OVERLAPPABLE
pragma like so:

    instance {-# OVERLAPPABLE #-} FromField a => FromField (Maybe a) where

This causes clients of the library that previously compiled (e.g.
the music-parts package) to no longer compile, due to a now
lacking OVERLAPPING pragma in their code.

The issue here is I'm trying to the right thing (move to new pragmas), but
that causes clients to fail to compile. My question is: how do we avoid
that? Would it be OK if they added the OVERLAPPING pragma first and then I
change my library to use OVERLAPPABLE?

On Tue, Aug 25, 2015 at 1:25 PM, Simon Peyton Jones <simonpj at microsoft.com>
wrote:

> What's the right way to migrate code? Just switching my library to the new
> pragmas breaks code, so that doesn't seem very attractive.
>
>
>
> I don’t understand.  Can you describe the problem more precisely, perhaps
> with an example?
>
>
>
> S
>
>
>
>
>
> *From:* ghc-devs [mailto:ghc-devs-bounces at haskell.org] *On Behalf Of *Johan
> Tibell
> *Sent:* 25 August 2015 10:42
> *To:* ghc-devs at haskell.org
> *Subject:* OVERLAPPABLE/OVERLAPPING/OVERLAPS pragmas are confusing
>
>
>
> It was brought to my attention that cassava, my library,
> uses OverlappingInstances, which is now deprecated. There's a suggested fix
> here: https://github.com/tibbe/cassava/pull/95.
>
>
>
> The fix seems correct but, as Mikhail points out, makes some client code
> no longer compile (due to a now missing OVERLAPPABLE pragma).
>
>
>
> What's the right way to migrate code? Just switching my library to the new
> pragmas breaks code, so that doesn't seem very attractive. Do clients have
> to migrate before the libraries they use?
>
>
>
> -- Johan
>
>
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/ghc-devs/attachments/20150825/9df45614/attachment.html>