[REBOL] Optional arguments
From: d4marcus::dtek::chalmers::se at: 17-May-2001 23:25
Long time ago, I recall there was a thread about handling optional
arguments. I didn't realise at the time that Rebol overall has just as
good support for this as LISP, but indeed it is different, and you also
have be a bit careful with the unset! values.
opt: func ['v] [either word? :v [if value? v [get :v]] [v]]
test: func [a [any-type!] b [any-type!] c [any-type!]] [
reduce [opt a opt b opt c]]
Now, the problem is to delimit the expression. Argument fetching is always
greedy so you have to do this somehow. But it proves to be easy.
>> print test 1 probe to-word "hi!" ; this won't work
hi!
** Script Error: hi! has no value
** Near: hi! none
>> print (test 1) probe to-word "hi!" ; this works
1 none none
hi!
== hi!
>> (print test 1) probe to-word "hi!" ; this works as well
; you can do without parens if you give all arguments
>> print test () None 1 'done
none none 1
== done
Instead of using parens (which resembles LISP a bit), you can (in good old
Rebol tradition) use blocks:
>> print do [test 1] probe to-word "hi!"
1 none none
hi!
== hi!
To my surprise, though the docs mention how to delimit expressions in
regards of changing the order of evaluation, i.e. for math expr:
>> 1 + 2 * 3
== 9
>> 1 + (2 * 3)
== 7
, it doesn't mention that it can be used for functions with optional
arguments as well. I think this ought to be included. At the moment I
figure such functions are used mostly on the console, but there's really
no reason why they couldn't be used in scripts too. You just have to
remember to delimit them.
A little LISP can be useful at times. :-)
Marcus
------------------------------------
If you find that life spits on you
calm down and pretend it's raining
