Eli Barzilay wrote:
About a minute ago, Carl Eastlund wrote:
On Thu, Nov 11, 2010 at 1:14 PM, Eli Barzilay <[email protected]> wrote:
So, you advocate eliminating `member' etc for predicates (why I
started with saying that this is much stickier than just adding a
`member?'). So we started with
(cdr (assq x alist))
then Scheme upgraded this to
(cond [(assq x alist) => cdr]
[else #f])
and you suggest continuing with
(cond #:with (define p (assq x alist))
[(pair? p) (cdr p)]
[else #f])
or, with more well-behaved proper sub lists
(cond #:with (define p (assq x alist))
[(not (eq? p #f)) (cadr p)]
[else #f])
(Yes, you can still use `pair?' but that would be ugly for the same
reason.)
I also wonder how many newbies (or people that just want to type less)
will fall into traps like
(if (member? x l)
(+ 1 (find x l))
0)
My loud "ugh" should be expected now.
(dict-ref x alist)
The above is "an example", substitute appropriately.
Do you have other examples in mind? Truthiness values are a substitute
Maybe monad, so we could potentially find it anywhere Haskell types use
Maybe. Regexp matching? Where else?
Truthiness values require nonlocal changes to deal with the None (#f)
case, just like the Maybe monad does. Sounds like a good reason to avoid
them, and stick to higher-level control, or passing to "value-missing"
thunks like in hash-ref.
Also, I can read this only because I'm already insane:
(cond [(assq x alist) => cdr]
[else #f])
Neil T
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