[Haskell-cafe] Where are rank-3 types necessary in practice for maintaining abstraction?

[Timotej Tomandl]

Hello,

So we need rank-2 type in runST :: (forall s. ST
<https://hackage.haskell.org/package/base-4.11.0.0/docs/Control-Monad-ST.html#t:ST>
s a) -> a, to prevent s from appearing in a.

I have been thinking about this for a bit, but I failed to come up with a
practical situation, where rank-3 types are necessary for safety of some
abstraction.

The rank-3 example in here and any other I found, look very synthetic, i.e.
limiting computation to id:
https://ocharles.org.uk/blog/guest-posts/2014-12-18-rank-n-types.html
and compared to the runST example of limiting a scope of a type variable
for purposes of safety looks unnatural.
Could anyone please point me to a practical example of rank-3 polymorphism,
where it is necessary for safety of an abstraction, if it exists?

I suspect there is a situation, where rank-3 is necessary for maintaining
abstration exists, but I can't think of any.
Any ideas about such situations and even better situations where this is
used on hackage?

Timotej Tomandl
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