Proposal: Add conspicuously missing Functor instances for tuples

Eric Seidel eric at
Mon Jan 18 21:31:13 UTC 2016

On Mon, Jan 18, 2016, at 13:22, Henning Thielemann wrote:
> On Mon, 18 Jan 2016, John Wiegley wrote:
> >>>>>> Henning Thielemann <lemming at> writes:
> >
> >> I would never use such an instance. Can I be warned if I accidentally use it
> >> anyway?
> >
> > I don't know why "I wouldn't use it" should extend to "it shouldn't exist".
> Because I cannot turn it off and I cannot get warning. It is inserted by 
> GHC whenever it matches, even if I made a mistake when programming.

There may be an easier way, but one could write a Core-to-Core plugin
that kills the build if it sees an expression like `fmap @(,)`.

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