Haskell Foldable Wats

Mark Roberts markandrusroberts at gmail.com
Thu Feb 25 18:26:02 UTC 2016

Perhaps such warnings could be added to a tool like HLint? I do not think
they belong in GHC.

On Thu, Feb 25, 2016 at 5:40 AM, Augustsson, Lennart <
Lennart.Augustsson at sc.com> wrote:

> Yes, at this point I think the best we can do is to make sure we get good
> warnings.
> I'd suggest that we can attach a warning to a method in each instance.
> So when using length on a tuple we'd get a warning.
> I don't even want to upgrade ghc to get the FTP until such a warning is in
> place (we are still on 7.8).
> -----Original Message-----
> From: Libraries [mailto:libraries-bounces at haskell.org] On Behalf Of Phil
> Ruffwind
> Sent: 25 February 2016 13:01
> To: Manuel Gómez
> Cc: Haskell Libraries
> Subject: Re: Haskell Foldable Wats
> > 2.  to implement a language extension in GHC that allows source
> > language to specify a certain instance will never be defined and a
> > type error shall be thrown statically at any use site, or
> How about a feature that allows GHC to warn whenever a certain instance is
> used? That way, if you don't like a certain instance, you can put {-#
> FORBIDINSTANCE Foldable ((,) a) #-} in your modules and get warnings
> whenever that instance is used.
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