Re: optional arguments in CLush functions?

On Tuesday 15 November 2005 07:56 pm, Ralf Juengling wrote:
> Hi,
> 
> When I write C functions where the arguments are objects
> (pointers) and want to make some of the arguments optional,
> an easy way to do so is by the convention that NULL pointers
> indicate absent arguments.
> 
> I'd like to do something similar with CLush. For the sake
> of an example, say I might want to write my own vector copying
> routine and if three args are provided, then the last arg is
> taken to be the storage for the result.
>
> Of course, it doesn't work currently--at least not the
> way I tried. There are a number of questions here:
No the compiler does not support this.
 
> 1. Is there any way, currently, to do what I want to do?
Use the 'overload' facility...
Exemple:

(libload "overload")
 [overload.lsh]
? (de f1(a) ((-flt-) a) (* a a )))
= "/home/leonb/lush/lsh/libstd/overload.lsh"
= f1
? (de f2(a b) ((-flt-) a b) (+ (* a a)(* b b))))
= f2
? (dhc-make () f1 f2)
Preprocessing and parsing f1 ...
Preprocessing and parsing f2 ...
Generating C for f1 ...
Generating C for f2 ...
gcc -DHAVE_CONFIG_H   -DNO_DEBUG -Wall -O3 -march=i686 -mmmx -msse -pthread .....
? (overload f f1 f2)
= ::DM:883fcf4
? (f 2)
= 4
? (f 3 4)
= 25

Note that <f> compiles, but is slow in interpreted mode
because it uses lisp code to match its arguments 
and their types.  When <f> is compiled, the
overload resolution happens in the compiler...

> 2. When you try to create a NULL ptr (a gptr object with
>     value NULL) in lush, the result is nil. When the CLush
>     compiler encounters a nil, it considers it a boolean
>     value rather than a NULL pointer.
Well, if you declare that the argument is a gptr, it will take it as a null pointer.
The problem is that you cannot declare that a variable can be either a null gptr, or another object.
This is a limitation of the type system.  The overload facility suggests how we could
handle this at compile time, but the type system in the compiler does not support this.

> 3. Would it be possible to change the CLush compiler to
>     make it accept a NULL ptr as arguments where the
>     corresponding formal parameter is an object?
Not without major redesign.


- L.



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