[plt-scheme] HtDP

On Mon, Jul 7, 2008 at 6:11 PM, dave yrueta <dyrueta at gmail.com> wrote:
>
> (define(fold f th lox)
>  (cond
>    [(empty? lox)th]
>    [else
>     (f (first lox)
>        (fold f th (rest lox)))]))
>

What is the contract for fold?

>
> (define(mapp2 f lox)
>  (fold (cons f) empty lox))
>
> This won't work, because (cons f) is not a legitimate expression.  I
> started work on next problem in the meantime, and that seems even more
> puzzling, so I feel like there is some basic concept about abstraction
> that I'm not getting.  Any suggestions?
>

Here's where a contract is going to be useful for you. What does fold
expect as arguments? Certainly, (cons f) is not something fold is
expecting as its first argument.

Cheers,

Marco