On Tue, Sep 6, 2016 at 7:30 PM, Sanjeev Sharma <throw...@gmail.com> wrote:
> Thanks, that's helpful
>
> I am having trouble coming up with a quick and easy shorthand to recur on
> the rest parameter - is there a standard way to cdr down this list without
> an intermediary helper function that takes out the raise-nesting-depth
> effect?
>
> Recursion on the base define (the one with the rest parameter) always adds
> another layer of nesting for the rest parameter - the first time recurring
> one must cdr, all the other times one must fiddle with car & cdr and what
> I'm doing has been prone working for the first few iterations but
> eventually arity-mismatches.
>
>
>From your description, the arity mismatches are probably caused by the fact
that you're defining `subsets` as a variable arity function, but internally
the recursive uses assume that it takes a single list. Your code in the
original post was:
(define ss
(lambda s
(if (null? s)
(list nil)
(let ((rest (subsets (cdr s))))
(append
rest
(map
(lambda (x)
(append (list (car s))
x))
rest))))))
The `(lambda s ...)` is variable-arity, but when you call `ss` internally,
you're always passing a single list as the argument. If you want a
variable-arity version, the simplest way is to define it as a simple
wrapper around a recursive fixed-arity version. Alternatively, you could
use `apply` in your recursive calls, but I'd call that bad style.
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