On Monday, November 9, 2015 at 3:17:59 AM UTC, Neil Van Dyke wrote:
> Note that, unlike some languages, I *don't* want `x` and `y` to be
> optionally positional -- only keyworded.
In my opinion, supplying a default value should make an argument implicitly
keywordable. So (define (a b c (d 3) (e 4)) ...) would have 2 required
positional arguments, and d and e could be present or omitted. If an argument
has a default, it means it should be possible to omit it, and if you cannot
specify keywords for the arguments after it (with defaults) then you cannot
omit it. That leads to things like this:
(define (foo a b (c #f) (d null) (e null)))
(list a b c d e))
(let ((e '(1 2 3)))
(list
(foo 1 2 #f null e)
(foo 1 2 #f null '(2 3 4))
(foo 1 2 #f e e)
(foo 1 2 #f e)))
Notice how in all cases the "optional" argument c had to be explicitly
specified with its own default value. That is confusing because people can't
tell if you mean "just ignore this and use the default" or "I really do think
it important that c have this value."
So, optional /non-keywordable/ arguments are a bad idea, in my opinion. Being
able to do something like this is very important for clarity:
(let ((e '(1 2 3)))
(list
(foo 1 2 :e e)
(foo 1 2 :e '(2 3 4))
(foo 1 2 :d e :e e)
(foo 1 2 :d e)))
I don't have a real preference whether d and e should be allowed to be
/positional/ though. It seems reasonable that it might be confusing to go (a 1
2 3 4) where either 3 or 4 are optional arguments with defaults if omitted. A
case-lambda would be the best form if you really want (a 1 2) (a 1 2 3) and (a
1 2 3 4). But on the other hand, it doesn't seem too bad, as long as I have the
option of specifying those optional arguments as keywords.
But I don't have that option, because I have to say "#:keyword-argument
(keyword-argument default)" or it doesn't /let/ me use a keyword to specify
that argument's value. That leads to me being forced to boilerplate in defaults
for intermediary optional variables. So, that's my big beef with the
lambda/define syntax.
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