> On Apr 20, 2017, at 10:04 PM, Lawrence Bottorff <borg...@gmail.com> wrote:
>
> Looking at HTDP-1
> (http://htdp.org/2003-09-26/Book/curriculum-Z-H-25.html#node_chap_19), I see
> this:
>
> (define (filter predicate alon)
> (cond
> [(empty? alon) empty]
> [else (cond
> [(predicate (first alon))
> (cons (first alon)
> (filter predicate (rest alon)))]
> [else
> (filter predicate (rest alon))])]))
>
> Then to find if a member of a list l is below a number i
>
> (define (below i l)
> (local ((define (below-i x)
> (> i x)))
> (filter below-i l)))
>
> As the proverbial beginner -- who doesn't totally understand closures -- this
> looks like a use of the closure idea. Right? The i is "magically" known in
> `filter`, which seems pretty closure-ish to me. Or am I totally wrong?
Here is the code in fixed width font:
(define (below i l)
(local ((define (below-i x)
(> i x)))
(filter below-i l)))
i is valid in the entire function body of below: (local . . .) [This is called
scope; see intermezzo]
so the i inside of the inner function definition (below-i) refers to the i in
the outer function def.
So when filter calls below-i, this function really knows what value i is.
In this sense, below-i is a closure, which is a pair of two things: the
variables it knows (and their values) PLUS the code it can run.
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