Take a look at this PR:
https://github.com/haskell/primitive/pull/109/files
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
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 _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.