[swift-users] Why can I not filter or map a dictionary to another dictionary?

Dave Abrahams dabrahams at apple.com
Thu Oct 27 00:23:23 CDT 2016

on Wed Oct 26 2016, Rick Mann <swift-users-AT-swift.org> wrote:

> It seems fairly natural to want to do this:
> let bigDictionary = ...
> let smallerDictionary = bigDictionary.filter { key, value in <some test returning Bool> }

> Similarly, it seems natural to want to map this way.

It's reasonable for filter, but maybe not for map.  Map follows certain
laws, and one of them is that you get the same number of elements out as
you put in.  But you can easily map all keys to the same key, and the
law would be violated.


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