Ch. 19: discuss while let referencing ch. 17

This is no longer the first time people see `while let` in the book, so
(a) update the text to mention Chapter 17, and (b) change the example
slightly so it shows something other than the `Option`-based scenario
readers will have already seen in much more interesting ways back in the
earlier part of the book.

src/ch17-02-concurrency-with-async.md

@@ -242,8 +242,6 @@ loop is the loop version of the `if let` construct we saw back in Chapter 6. The
 loop will continue executing as long as the pattern it specifies continues to
 match the value.
 
-<!-- TODO: update text in ch. 19 to account for our having introduced this. -->
-
 The `rx.recv` call produces a `Future`, which we await. The runtime will pause
 the `Future` until it is ready. Once a message arrives, the future will resolve
 to `Some(message)`, as many times as a message arrives. When the channel closes,

src/ch19-01-all-the-places-for-patterns.md

@@ -98,9 +98,13 @@ not alert us to the possible logic bug.
 ### `while let` Conditional Loops
 
 Similar in construction to `if let`, the `while let` conditional loop allows a
-`while` loop to run for as long as a pattern continues to match. In Listing
-19-2 we code a `while let` loop that uses a vector as a stack and prints the
-values in the vector in the opposite order in which they were pushed.
+`while` loop to run for as long as a pattern continues to match. We first saw a
+`while let` loop in Chapter 17, where we used it to keep looping as long as a
+stream produced new values. Similarly, in Listing 19-2 we code a `while let`
+loop that uses a vector as a stack and prints the values in the vector in the
+opposite order in which they were pushed. We also use a new helper function,
+`ok_or`, which converts an `Option` into a `Result`, to show how `while let` can
+work with any valid pattern.
 
 <Listing number="19-2" caption="Using a `while let` loop to print values for as long as `stack.pop()` returns `Some`">
 
@@ -112,9 +116,11 @@ values in the vector in the opposite order in which they were pushed.
 
 This example prints 3, 2, and then 1. The `pop` method takes the last element
 out of the vector and returns `Some(value)`. If the vector is empty, `pop`
-returns `None`. The `while` loop continues running the code in its block as
-long as `pop` returns `Some`. When `pop` returns `None`, the loop stops. We can
-use `while let` to pop every element off our stack.
+returns `None`. The `ok_or` function turns `Some(value)` into an `Ok(value)` and
+turns the `None` for the empty vector into an `Err("empty!")`. The `while` loop
+continues running the code in its block as long as `ok_or` returns `Ok`. When
+`ok_or` returns `Err("empty!")`, the loop stops. We can use `while let` to pop
+every element off our stack.
 
 ### `for` Loops