Haskell Foldable Wats

[Augustsson, Lennart]

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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