[swift-evolution] [Review] SE-0142: Permit where clauses to constrain associated types

[Austin Zheng]

* What is your evaluation of the proposal?

+1. I very much want to see this in Swift, and it seems like a logical
progression that has the potential to eliminate ugly workarounds.

* Is the problem being addressed significant enough to warrant a change to
Swift?

Yes. Right now the desired relationships between the associated types of a
protocol must be written out at each site of use (for example, a generic
function declaration that involves that protocol type). When defining a
type that conforms to such a protocol these relationships must either be
discovered through documentation, or by examining the APIs with which the
conforming type will be used.

By changing this implicit contract (through documentation + use site
constraints) into an explicit contract (through constraints directly
expressed at the point where the associated types are defined), programmers
seeking to write conforming types have an easier time understanding how the
protocols they are conforming to are intended to work, and programmers
seeking to write APIs involving those protocols don't need to spell out the
constraints repeatedly.

* Does this proposal fit well with the feel and direction of Swift?

Yes. We already have a pretty regular regime for the use of `where` to
define constraints, for example in generic type and function declarations
and when defining constrained extensions. Adding support for `where` to
associated types would be a natural extension of the existing ability to
specify protocol conformance, and its semantics would not be surprising to
those familiar with the other uses of `where`. (This comment I think
applies to all three of the proposed syntaxes.)

* If you have used other languages or libraries with a similar feature, how
do you feel that this proposal compares to those?

n/a

* How much effort did you put into your review? A glance, a quick reading,
or an in-depth study?

Read the review, followed most of the pertinent threads over the past few
months with varying degrees of attentiveness.
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