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.
Sorry for the spam.
On Tue, Apr 3, 2018 at 4:36 AM, 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... 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