[plt-scheme] HTDP 21.2

On Apr 10, 2007, at 4:51 PM, Don Blaheta wrote:

> I'm either looking to report a few errata or curious to hear an
> explanation.  In section 21.2 of HtDP (p 313 in the dead tree  
> version),
> a number of higher-order functions are listed as being defined in
> Scheme, and by implication, in the language levels associated with the
> book.
>
> One is (quicksort cmp alox), even in the most-updated online version;
> but that function is listed in the library documentation as deprecated
> in favour of (sort cmp alox).  Which isn't a huge problem, except that
> the language-level Scheme doesn't seem to have sort defined.


> Of somewhat greater concern is that (assf p? alop) is listed in this
> figure, but is no longer present in any of the language levels.  A
> couple of my students have hounded me about this; one of the  
> problems I
> had written was on an alist, and I'd explicitly pointed them at the
> various alist functions (some, like assq, *are* defined).  I ended up
> just telling them to write assf themselves as an exercise, but I'm
> wondering why it got taken out (if indeed that was even  
> intentional)....

[It was intentional.]

Sorry for the inconsistencies. I haven't updated the on-line book in
a couple of years and that's clearly a problem. But I will be on a
sabbatical soon and I am hoping that I'll be able to show something  
for it.

-- Matthias