Proposal: Add common instances for Control.Applicative.Const

Edward Kmett ekmett at
Mon Oct 14 18:32:28 UTC 2013

There is a somewhat weak prescriptive argument that one shouldn't use these
things for long term usage, e.g. that you put, say a WrappedMonad shim on
just for long enough to pass something and get a different dictionary, but
really the result is it just makes code harder to test, debug and work with
at a REPL and raises the question of what components should people be using
when they DO need to embed such a thing in a place where it winds up
visible or stored in a container.

I'm generally positive on adding these kinds of instances, especially where
they don't require us to go out and lean on UndecidableInstances and the


On Mon, Oct 14, 2013 at 1:55 PM, David Luposchainsky <
dluposchainsky at> wrote:

> On 2013-10-14 18:33, Eric Mertens wrote:
> > It would be handy for Control.Applicative.Const to have the various
> > derivable instances (Eq, Ord, Read, Show).
> Let me ask the other way round: is there ever a reason to *not* add
> these instances if they're possible and unambiguous?
> David
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