Re: [Swig-devel] [MATLAB] How to handle "ans" output?

[Joel A. <j.a...@gm...>]

Hi!

After some thinking, I would like to modify my suggestion: It should be
possible to check if _all_ the overloads of a particular (member) function
has at least one output argument. And for those functions, enforce an "ans"
output.

That is, if a function has two overloads, and at least one of them void, no
"would be produced":

void foo(double a, double b);
double foo(double a); // No "ans" produced!

But if _all_ overloads have non-void returns, it an "ans" will be produced:

double foo(double a, double b); // "ans" produced
double foo(double a);  // "ans "produced

I think the normal situation is that all the overloads are either non-void
or void and this will be covered by this. It will also not add any
noticeable overhead or larger wrapper files. Nor do we have to worry about
strange "[]" returns from void functions.

What say you?
Joel

P.S.: I could take a shot at the implementation, if we agree on a design.

2014-08-19 20:51 GMT+02:00 Kris Thielemans <kri...@gm...>:

> From: Joel Andersson  Sent: 19 August 2014 11:29
> >
> > There is one thing I find particularly annoying when playing around with
> CasADi
> > from MATLAB; the fact that the "ans" argument in MATLAB is not generated
> > when you want it.
> >
>
> Yes, this is quite annoying, I agree.
>
> > A function such as arcsinh in CasADi is now wrapped as follows:
> > function varargout = function_name(varargin)
> >   [varargout{1:nargout}] =
> > casadi_coreMATLAB_wrap(1532,'SX_arcsinh',self,varargin{:});
> > end
> >
> > The problem is that if you type "asinh(x)" in MATLAB, nargout will be
> zero, so
> > nothing will be returned from the wrapper. To get an "ans" argument, you
> need
> > to replace "nargout" with 1, or more generally with max(nargout,1):
> > function varargout = function_name(varargin)
> >   [varargout{1:max(1,nargout)}] =
> > casadi_coreMATLAB_wrap(1532,'SX_arcsinh',self,varargin{:});
> > end
> >
> > My proposed resolution is to always return at least one output whenever
> any
> > overload of the function has any outputs arguments. This means that if
> you
> > have two overloaded functions, one without outputs and another with
> outputs, a
> > dummy output (0-by-0 matrix) would be returned from the overload with no
> > outputs:
> >
> > void foo(double a); // no outputs, but dummy output [] returned
> > int foo(double a, double b); // outputs
> >
> > Does this sound reasonable? In a MATLAB script, you'd put a semicolon
> after
> > foo(a) anyway, preventing the creation of [].
> >
>
> I don't find it 100% satisfactory (it's not entirely clean, and people
> might not automatically put the semicolon if the function shouldn't return
> anything) but it's definitely a better solution than what we have at
> present. Also, the above example is somewhat contrived (anyone making an
> API like that is asking for confusion) as normally the overloading happens
> based on the input arguments, but I guess we have to cover corner cases as
> well.
>
> I have no clear idea how you'd implement it though. When wrapping the
> code, you'd need to know which functions return void, and if so, return [],
> no? If so, I guess we could just as well modify the matlab class as well?
> (ok, that'd seems to mean we need to transport a lot of complication into
> the matlab code which we don't have now, so probably not a good idea).
>
> Another idea:
> As opposed to returning [], return a special object (e.g. SwigVoid). Once
> we do that, a few different options:
> - set its display properties to the empty string. So effectively you want
> see it. (hmmm)
> - let the matlab wrapper detect that. Something like this (not checked!)
>
> function varargout= swig_return(varargin)
> (
>   If (nargout==0 && nargin==1 && class(varargin{1}, 'SwigVoid'))
>    return;
>   else
>     varargout = varargin;
>     return;
> )
>
> function varargout = function_name(varargin)
>    [varargout{1:nargout}] = swig_return(...
>      casadi_coreMATLAB_wrap(1532,'SX_arcsinh',self,varargin{:}));
>   end
>
> if we then make swig_return a method of SwigRef, it could work. Or indeed,
> make a method swig_call_wrapper that does this all in one go...
>
> not sure if this can work though
>
> Kris
>
>


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