[swift-evolution] [swift-evolution-announce] [Review] SE-0089: Replace protocol<P1, P2> syntax with Any<P1, P2>

[Dave Abrahams]

on Sat Jun 11 2016, Jonathan Hull <jhull-AT-gbis.com> wrote:

>> If your code has many manual type erasing wrappers corresponding to
>> protocols with associated types and/or Self requirements that also never
>> have to trap type mismatches, that would certainly be instructive
>> empirical data.  Would you care to share the protocols and wrappers you
>> are talking about?
> This code is a bit embarrassing (I wrote most of it as I was still
> learning Swift), but if it is helpful I will share:
> https://gist.github.com/jonhull/639e756ad5228348f93f40f06169588c
>
> It doesn’t trap anywhere (that I know about). Some of the code which
> calls it does throw an error in the case of mismatched types (but it
> doesn’t/shouldn't crash).  

If that error is being thrown to handle what you consider to be a
programming error, that is the moral equivalent of a trap.

> Most functions which use it are generic on the associatedType.
>
> It does work fairly well though (even on the AppleWatch).  The main
> issue is that I have no way to persist the values which have been
> drawn into the type-erased world (or even the value-type world,
> really).
>
> Also, if anyone has a better way to write the type erasing wrapper, I
> would love to hear it. These are very memory intensive…

It looks like you could make some gains by, instead of storing a closure
for each operation, using a base class with a method for each operation,
per the box types in
https://github.com/apple/swift/blob/master/stdlib/public/core/ExistentialCollection.swift.gyb

HTH,
-- 
Dave