On Aug 28, 2011, at 4:36 PM, Allen Wirfs-Brock wrote:
> On Aug 27, 2011, at 6:12 AM, Andreas Rossberg wrote:
>
>> On 27 August 2011 00:34, Allen Wirfs-Brock <[email protected]> wrote:
>>> If the formal parameter list includes any non-optional, non-rest formal
>>> parameters, the length is the total number of non-optional/non-rest formal
>>> parameters.
>>> For example:
>>> function bar(a,b,c) {} //bar.length=3
>>> function baz(a,b,c=0) {} //baz.length=2
>>> function bam(a,b,c=0,...d) {} //bam.length==2 BTW, is this legal?
>>
>> Makes sense. (And yes, I don't see why the latter shouldn't be legal.)
> Because the is a potential for misinterpreting the user intent on such a
> call. For
> bam('a','b',1,2,3)
> we surely have to interpret the argument/parameter mapping as:
> a='a',b='b',c=1,d=[2,3]
> but it is easy to imagine a programmer intending
> a='a',b='b',c=0,d=[1,2,3]
> Making it illegal to have a formal parameter list that has both optional and
> result parameters might result the likelihood of that confusion,
That's too restrictive. There are lots of ways to go wrong in JS, we don't try
to put programmers in a five-point harness.
Python:
>>> def foo(a,b,c=1,*d):
... print(a, b, c, d)
...
>>> foo(0,1,2,3,4)
0 1 2 (3, 4)
>>> foo(0,1,2,3)
0 1 2 (3,)
>>> foo(0,1,2)
0 1 2 ()
>>> foo(0,1)
0 1 1 ()
(Python reflects rest params [splats] as tuples not lists.)
Ruby is of course pretty similar:
def foo(a,b,c=1,*d)
print a, b, c, d, "\n"
end
foo(0,1,2,3,4)
foo(0,1,2,3)
foo(0,1,2)
foo(0,1)
^D
01234
0123
012
011
(My Ruby REPL skills are sad -- ugly run-together stdout follows the TTY EOF ^D
above.)
>>> If there are no non-optional or non-rest formal parameters the length is 1.
>>> function bar1(a=0) {} //bar1.length=1
>>> function baz1(a=0,b=1,c=2) {} //baz1.length=1
>>> function bam1(...a) {} //bam1.length==1
>>
>> I'm not so sure about this, it seems incoherent. Why not 0, especially
>> for the first two? You mentioned builtins like Array above, but I
>> would rather count them as the exception to the rule (especially given
>> that the builtins don't seem entirely consistent wrt length anyway).
>
> In my proposal, I am decided to make a clear distinction between truly empty
> formal parameter lists and those with only various forms of optional formal
> parameters by only giving a 0 length to the empty formals case. That's a
> debatable decision but it seems desirable to distinguish the two cases and
> the built-ins is the only precedent that we have to follow.
You make a distinction but create an ambiguity between
function foo(a=0)... // length 1
function bar(a=0,b=1) // length 1
That seems no more consistent (and less coherent as Andreas put it). If
trailing optional parameters do not contribute to length, then we have a
consistent single rule for length. That it does not match all built-ins is sad
history, not to be rewritten lightly but not to overdetermine the future.
> I'm not sure if there is any real use case for the length property of
> ECMAScript functions. Does anybody know of one? Regardless, I do think we
> can get rid of it.
Working on instrumentation to find uses on the web. More in a bit,
/be
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