# Forum: Ruby True arity

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on 2007-05-29 13:13
```Is there a way to get the *maximum* number of arguments that a method
can receive? Method#arity only gives the lower bound...

Daniel```
on 2007-05-29 13:44
```On 5/29/07, Daniel DeLorme <dan-ml@dan42.com> wrote:
> Is there a way to get the *maximum* number of arguments that a method
> can receive? Method#arity only gives the lower bound...

I am afraid I fail to understand, what would you define as the maximum?

def a(a)...  I would say the maximum is 1 which is the arity
def b(*b)   arity is -1 but what would the maximum of parameters be?
def c(c,*d) arity = -2 but some question as above.

The question is, what would you like to achieve?

Cheers
Robert```
on 2007-05-30 00:27
```Robert Dober wrote:
> The question is, what would you like to achieve?
In the last 2 cases you give the maximum would be Infinity. But in a
case like this:
def foo(a, b=nil, c=nil)
the maximum would be 3 but arity only gives me -2 which means "this
method has 1 required argument". If I invoke the method like:
foo(*args)
I want to know for what size of args is this valid? It would be nice if
arity returned a Range object (1..3)

Daniel```
on 2007-05-30 04:23
```arity is not able to show you all the information you need, default
parameter are a curse in this regard (or the format arity uses - a
simple integer is not enough to show all cases... even a range
wouldn't, since
(1..(1.0/0.0))
# 1..Infinity
won't tell you that much (i think)
I don't know of a really simple programmatic way to determine the
_really_ really need it. Best would be to just raise and tell the user
he used the wrong arity. Or you could do a rundown, passing as many
parameters as possible and going down until it doesn't raise any more.

Please note that this post is full of bad practice and evil hacks :)

^ manveru```
on 2007-05-30 08:23
```On 5/29/07, Daniel DeLorme <dan-ml@dan42.com> wrote:
> >
> > The question is, what would you like to achieve?
>
> In the last 2 cases you give the maximum would be Infinity. But in a
> case like this:
>    def foo(a, b=nil, c=nil)
I know I missed something :(, indeed a good question, thx for
explaining to a dummy ;).
> the maximum would be 3 but arity only gives me -2 which means "this
> method has 1 required argument". If I invoke the method like:
>    foo(*args)
> I want to know for what size of args is this valid? It would be nice if
> arity returned a Range object (1..3)
>
> Daniel
>
>
Cheers
Robert```
on 2007-05-30 10:57
```Daniel DeLorme wrote:
> In the last 2 cases you give the maximum would be Infinity. But in a
> case like this:
>   def foo(a, b=nil, c=nil)
> the maximum would be 3 but arity only gives me -2 which means "this
> method has 1 required argument". If I invoke the method like:
>   foo(*args)
> I want to know for what size of args is this valid? It would be nice if
> arity returned a Range object (1..3)

It could be done, but the current Ruby implementations don't provide
this information. Of course, all of them could...we know how many
required arguments there are, we know how many optional arguments there
are, and we know if there's a "rest" arg or not.

Maximum =

...base arity if no optional args or rest arg
...base arity plus optional arg count if no rest arg
...infinity if rest arg in any case

Minimum = ...well you know it, because we have this today.

I guess the question would be whether this is useful enough information
to warrant a semantic change in "arity" representation.

- Charlie```
on 2007-05-30 11:52
```Charles Oliver Nutter wrote:
> I guess the question would be whether this is useful enough information
> to warrant a semantic change in "arity" representation.

It can be useful in some cases and it is *certainly* cleaner than the
wondrous hack that is a negative arity, but I'm never very fond of
backwards compatibility breakage. I'd be more in favor of introducing a
new method (e.g. 'arityrange') that would return a Range object. (please

IMHO, Range objects are underused. MatchData#offset would be so much
nicer if it returned (a...b) instead of [a,b]

just my 2¥

Daniel```
on 2007-05-30 13:55
```On 5/30/07, Daniel DeLorme <dan-ml@dan42.com> wrote:
> IMHO, Range objects are underused. MatchData#offset would be so much
> nicer if it returned (a...b) instead of [a,b]
>
> just my 2¥
that is clever Daniel ;)

but seriously I just wanted to backup your idea which seems pretty