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

David Feuer david.feuer at gmail.com
Wed Apr 4 03:35:35 UTC 2018

Take a look at this PR:


The heterogeneous array creation functions I propose take rank-2 traversal
functions as arguments and are therefore rank-3. In this case, the reason
is a bit boring: the package in question can't depend on either (any?) of
the packages offering rank-2 versions of Traversable.

On Tue, Apr 3, 2018, 6:37 AM Timotej Tomandl <timotomandl at gmail.com> wrote:

> Hello,
> So we need rank-2 type in runST :: (forall s. ST
> <https://hackage.haskell.org/package/base->
> 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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