get nargs for a function

get nargs for a function

[Sam Halliday]

Hi all,

Is there a built in way to return the arg list (actually just the number of args) for a function that has been passed to me?

If I was just in dynamic scope, I could get cadr and count the size, but I have to support closure as well which is caddr (and however that changes in the future).

Best regards,
Sam

Re: get nargs for a function

[Ian Zimmerman]

On 2015-08-28 09:55 -0700, Sam Halliday wrote:

> Is there a built in way to return the arg list (actually just the
> number of args) for a function that has been passed to me?
 
> If I was just in dynamic scope, I could get cadr and count the size,
> but I have to support closure as well which is caddr (and however that
> changes in the future).

How does advice do it?

-- 
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Re: get nargs for a function

[Stefan Monnier]

> Is there a built in way to return the arg list (actually just the number of
> args) for a function that has been passed to me?

The answer is "yes, but fundamentally no".
The reason it's fundamentally no, is that you'd need to answer questions
such as "how many arguments does cl-member take".

In most cases, the better approach is to do something else, such as:

    (condition-case
        (funcall thefunction arg1 arg2 ...)
      (wrong-number-of-arguments
        ...do-something-else...))


-- Stefan

Re: get nargs for a function

[Sam Halliday]

On Saturday, 29 August 2015 22:41:49 UTC+1, Stefan Monnier  wrote:
> > Is there a built in way to return the arg list (actually just the number of
> > args) for a function that has been passed to me?
> 
> The answer is "yes, but fundamentally no".
> The reason it's fundamentally no, is that you'd need to answer questions
> such as "how many arguments does cl-member take".
> 
> In most cases, the better approach is to do something else, such as:
> 
>     (condition-case
>         (funcall thefunction arg1 arg2 ...)
>       (wrong-number-of-arguments
>         ...do-something-else...))

This is only to answer a toy problem anyway so good to know this doesn't scale. I ended up doing this

(defun args (function)
  "The arg list of FUNCTION."
  (let ((kind (car function)))
    (if (eq kind 'closure)
        (cadr (cdr function))
      (cadr function))))

because the nature of the problem required working on a function with an arbitrary argument list.

https://github.com/fommil/e99/blob/master/48.el

I suppose I could re-interpret the question to take the nargs explicitly.

Re: get nargs for a function

[Stefan Monnier]

> (defun args (function)
>   "The arg list of FUNCTION."
>   (let ((kind (car function)))
>     (if (eq kind 'closure)
>         (cadr (cdr function))
>       (cadr function))))

To get the description of the arglist, you're better off using
`help-function-arglist'.


        Stefan

Re: get nargs for a function

[Sam Halliday]

On Sunday, 30 August 2015 03:22:46 UTC+1, Stefan Monnier  wrote:
> > (defun args (function)
> >   "The arg list of FUNCTION."
> >   (let ((kind (car function)))
> >     (if (eq kind 'closure)
> >         (cadr (cdr function))
> >       (cadr function))))
> 
> To get the description of the arglist, you're better off using
> `help-function-arglist'.

I don't believe that works here as the input is typically anonymous.

Re: get nargs for a function

[Stefan Monnier]

>> To get the description of the arglist, you're better off using
>> `help-function-arglist'.
> I don't believe that works here as the input is typically anonymous.

If it doesn't work, please report it as a bug.


        Stefan

Re: get nargs for a function

[Sam Halliday]

On Monday, 31 August 2015 01:17:46 UTC+1, Stefan Monnier  wrote:
> >> To get the description of the arglist, you're better off using
> >> `help-function-arglist'.
> > I don't believe that works here as the input is typically anonymous.
> 
> If it doesn't work, please report it as a bug.

It does work, I just misunderstood the function documentation (I thought it took the name of a function, not the function itself). This is pretty much exactly what I wanted. Thanks!