Would it be OK if they added the OVERLAPPING pragma first and then I change my library to use OVERLAPPABLE? I think so, yes. Does that not work? Is it bad? Do you think the semantics is wrong? Simon From: Johan Tibell [mailto:johan.tibell@gmail.com] Sent: 25 August 2015 13:19 To: Simon Peyton Jones Cc: ghc-devs@haskell.org Subject: Re: OVERLAPPABLE/OVERLAPPING/OVERLAPS pragmas are confusing 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 mailto:simonpj@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@haskell.orgmailto:ghc-devs-bounces@haskell.org] On Behalf Of Johan Tibell Sent: 25 August 2015 10:42 To: ghc-devs@haskell.orgmailto:ghc-devs@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

Johan, to summarize: 1. If an instance is marked as OVERLAPPABLE, then clients may overlap it without having any pragmas 2. If an instance is NOT marked OVERLAPPABLE, then clients may still overlap it, but then they have to use an explicit OVERLAPPING pragma. So you should either add OVERLAPPABLE to your library, and then clients don't need to do anything, or you should remove it, and require that clients add OVERLAPPING. Note that using this mechanism across modules can be quite error prone. For example, you have to be very careful not to use an OVERLAPPABLE instance in your library, as if you do parts of the program might end up using one instance, and other parts may end up using another instance---GHC has no way of knowing about overlapping instance in client libraries, so it will simply use the best possible *local* instance. -Iavor On Tue, Aug 25, 2015 at 8:46 AM, Mikhail Glushenkov < the.dead.shall.rise@gmail.com> wrote:

Hi,

On 25 August 2015 at 14:18, Johan Tibell wrote:

...

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.

No, it's not quite like that. Client code can start to break when {-# LANGUAGE OverlappingInstances #-} is removed, as happened with the music-parts package. Adding an OVERLAPPABLE pragma to cassava's code made that error go away.

Client code can usually work around the problem of missing OVERLAPPABLE pragmas in the library by adding OVERLAPPING pragmas to their instances. The reason I suggested bumping cassava's version is that there may be some places in cassava that still need new pragmas that I've overlooked.

If GHC had an option for detecting overlapping instances at definition site, that'd help, I think, since then it'd be easier to find instances that need new pragmas. _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs