Thanks for your comments; criticism is always welcome! And for your kind words about Pyret, though I actually find parenthetical syntax a bit easier to write. It's still taking me some effort to get used to the irregularities of non-parenthetical syntax.

I don't know why you say types are an afterthought. HtDP teaches a profoundly _typed_ discipline of programming. The datatype mechanism in PLAI continues this with more enforcement. And plai-typed is anything _but_ an "afterthought", right?

PAPL is written as if types work. We have an internal type-checker that I've been using off-and-on this semester (but it's still in development). As soon as it's ready, all programs in the book will pass through it and the book will be almost completely typed.

I'm not going to get into an argument here about whether it is better to start with static types. Though I somewhat share @noelwelsh's viewpoint, I think the full situation is far more complex and demands actual research — HCI research — as opposed to just matters of opinion (including mine).