There's an easy to read paper by Okasaki titled "Even higher-order functions for parsing or Why would anyone ever want to use a sixth-order function?". Unfortunatly I can't find a link to a non-paywalled version. It shows how parser combinators themselves use rank-3 types and how defining a monad instance requires rank-6.

On Tue, Apr 3, 2018 at 1:05 AM Joachim Durchholz <jo@durchholz.org> wrote:

Am 03.04.2018 um 08:54 schrieb Timotej Tomandl:
> Ok, I thought about it a bit more and realized mask in Control.Exception
> is the one where rank-3 is necessary, which is the example I was looking
> for.

Given your newly acquired insights, do you expect that there will be
ultimately a valid example for higher ranks?
Is there a theoretical limit?
Or a practical one? E.g. it might be too awkward to mentally handle
higher-rank polymorphism - or maybe there's no real mental difference
when dealing with higher-ranked polymorphism, I don't know because I
didn't have to deal with that yet, so I'm curious.

 > Sorry for the spam.

Actually it was interesting.

Regards,
Jo
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