Asp Forum - Passing variable keywords to another function

Mart van de Wege

7/18/2015 10:51:00 AM

Hi all,

I'm trying to build a constructor function that calls make-instance, but
the number of arguments to pass to make-instance is unknown and depends
on the number of parameters passed to the original function.

I can do this with a macro easily, but is it possible to do it with a
function?

If I try this:

(defclass testclass ()
((first-slot :initarg :first-slot)
(second-slot :initarg :second-slot)))

(defun make-test-class (&rest args)
(make-instance 'testclass args))

then (make-test-class :first-slot 1) will fail because it
passes args as a single list to make-instance; I need a way to easily
destructure a list with an unknown number of elements to pass it to
make-instance. The ,@ operator does this in a macro; how can I do the
same in a function?

Mart

--
"We will need a longer wall when the revolution comes."
--- AJS, quoting an uncertain source.

14 Answers

Pascal J. Bourguignon

7/18/2015 11:13:00 AM

Mart van de Wege <mvdwege@gmail.com> writes:

> Hi all,
>
> I'm trying to build a constructor function that calls make-instance, but
> the number of arguments to pass to make-instance is unknown and depends
> on the number of parameters passed to the original function.
>
> I can do this with a macro easily, but is it possible to do it with a
> function?
>
> If I try this:
>
> (defclass testclass ()
> ((first-slot :initarg :first-slot)
> (second-slot :initarg :second-slot)))
>
> (defun make-test-class (&rest args)
> (make-instance 'testclass args))

(defun make-test-class (&rest args &key)
(apply (function make-instance) 'testclass :allow-other-keys t args))

--
__Pascal Bourguignon__ http://www.informat...
â??The factory of the future will have only two employees, a man and a
dog. The man will be there to feed the dog. The dog will be there to
keep the man from touching the equipment.â? -- Carl Bass CEO Autodesk

Dimitri Fontaine

7/18/2015 11:15:00 AM

Mart van de Wege <mvdwege@gmail.com> writes:
> (defclass testclass ()
> ((first-slot :initarg :first-slot)
> (second-slot :initarg :second-slot)))
>
> (defun make-test-class (&rest args)
> (make-instance 'testclass args))

I think you want:

(defun make-test-class (&rest args)
(apply #' make-instance 'testclass args))

--
dim

Pascal Costanza

7/18/2015 11:30:00 AM

On 18/07/2015 12:50, Mart van de Wege wrote:
> Hi all,
>
> I'm trying to build a constructor function that calls make-instance, but
> the number of arguments to pass to make-instance is unknown and depends
> on the number of parameters passed to the original function.
>
> I can do this with a macro easily, but is it possible to do it with a
> function?
>
> If I try this:
>
> (defclass testclass ()
> ((first-slot :initarg :first-slot)
> (second-slot :initarg :second-slot)))
>
> (defun make-test-class (&rest args)
> (make-instance 'testclass args))

(defun make-test-class (&rest args)
(apply #'make-instance 'testclass args))

Also:

(defun make-test-class (&rest args &key first-slot second-slot)
(apply #'make-instance 'testclass
:first-slot (modify-value first-slot)
args))

etc.

Pascal

--
My website: http:/...
Common Lisp Document Repository: http://cdr.eu...
Closer to MOP & ContextL: http://common-lisp.net/proje...
The views expressed are my own, and not those of my employer.

gengyangcai

7/18/2015 12:23:00 PM

On Saturday, July 18, 2015 at 6:56:07 PM UTC+8, Mart van de Wege wrote:
> Hi all,
>
> I'm trying to build a constructor function that calls make-instance, but
> the number of arguments to pass to make-instance is unknown and depends
> on the number of parameters passed to the original function.
>
> I can do this with a macro easily, but is it possible to do it with a
> function?
>
> If I try this:
>
> (defclass testclass ()
> ((first-slot :initarg :first-slot)
> (second-slot :initarg :second-slot)))
>
> (defun make-test-class (&rest args)
> (make-instance 'testclass args))
>
> then (make-test-class :first-slot 1) will fail because it
> passes args as a single list to make-instance; I need a way to easily
> destructure a list with an unknown number of elements to pass it to
> make-instance. The ,@ operator does this in a macro; how can I do the
> same in a function?
>
> Mart
>
> --
> "We will need a longer wall when the revolution comes."
> --- AJS, quoting an uncertain source.

This is what I got when I ran your code :

CL-USER 1 > (defclass testclass ()
((first-slot :initarg :first-slot)
(second-slot :initarg :second-slot)))
#<STANDARD-CLASS TESTCLASS 2009FAA3>

CL-USER 2 >
(defun make-test-class (&rest args)
(make-instance 'testclass args))
MAKE-TEST-CLASS

Pascal J. Bourguignon

7/18/2015 12:24:00 PM

CAI GENGYANG <gengyangcai@gmail.com> writes:

> On Saturday, July 18, 2015 at 6:56:07 PM UTC+8, Mart van de Wege wrote:
>> Hi all,
>>
>> I'm trying to build a constructor function that calls make-instance, but
>> the number of arguments to pass to make-instance is unknown and depends
>> on the number of parameters passed to the original function.
>>
>> I can do this with a macro easily, but is it possible to do it with a
>> function?
>>
>> If I try this:
>>
>> (defclass testclass ()
>> ((first-slot :initarg :first-slot)
>> (second-slot :initarg :second-slot)))
>>
>> (defun make-test-class (&rest args)
>> (make-instance 'testclass args))
>>
>> then (make-test-class :first-slot 1) will fail because it
>> passes args as a single list to make-instance; I need a way to easily
>> destructure a list with an unknown number of elements to pass it to
>> make-instance. The ,@ operator does this in a macro; how can I do the
>> same in a function?
>>
>> Mart
>>
>> --
>> "We will need a longer wall when the revolution comes."
>> --- AJS, quoting an uncertain source.
>
> This is what I got when I ran your code :
>
> CL-USER 1 > (defclass testclass ()
> ((first-slot :initarg :first-slot)
> (second-slot :initarg :second-slot)))
> #<STANDARD-CLASS TESTCLASS 2009FAA3>
>
> CL-USER 2 >
> (defun make-test-class (&rest args)
> (make-instance 'testclass args))
> MAKE-TEST-CLASS

The obvious missing step, the one that will signal an error is:

(make-test-class)

--
__Pascal Bourguignon__ http://www.informat...
â??The factory of the future will have only two employees, a man and a
dog. The man will be there to feed the dog. The dog will be there to
keep the man from touching the equipment.â? -- Carl Bass CEO Autodesk

Mart van de Wege

7/18/2015 6:10:00 PM

"Pascal J. Bourguignon" <pjb@informatimago.com> writes:

> Mart van de Wege <mvdwege@gmail.com> writes:
>
>> Hi all,
>>
>> I'm trying to build a constructor function that calls make-instance, but
>> the number of arguments to pass to make-instance is unknown and depends
>> on the number of parameters passed to the original function.
>>
>> I can do this with a macro easily, but is it possible to do it with a
>> function?
>>
>> If I try this:
>>
>> (defclass testclass ()
>> ((first-slot :initarg :first-slot)
>> (second-slot :initarg :second-slot)))
>>
>> (defun make-test-class (&rest args)
>> (make-instance 'testclass args))
>
> (defun make-test-class (&rest args &key)
> (apply (function make-instance) 'testclass :allow-other-keys t args))

d'oh.

Obvious once you see it.

Mart

--
"We will need a longer wall when the revolution comes."
--- AJS, quoting an uncertain source.

Kenneth Tilton

7/24/2015 5:42:00 PM

On Saturday, July 18, 2015 at 2:14:07 PM UTC-4, Mart van de Wege wrote:
> >> I'm trying to build a constructor function that calls make-instance, but
> >> the number of arguments to pass to make-instance is unknown and depends
> >> on the number of parameters passed to the original function.
> >>
> >> I can do this with a macro easily, but is it possible to do it with a
> >> function?

Don't do it at all. Sonya Keene was wrong. CLOS is very powerful and you lose all that power when you hide make-instance behind a maker function.

-hk

Mart van de Wege

7/24/2015 6:12:00 PM

Pascal J. Bourguignon

7/24/2015 6:14:00 PM

His Kennyness <kentilton@gmail.com> writes:

> On Saturday, July 18, 2015 at 2:14:07 PM UTC-4, Mart van de Wege wrote:
>> >> I'm trying to build a constructor function that calls make-instance, but
>> >> the number of arguments to pass to make-instance is unknown and depends
>> >> on the number of parameters passed to the original function.
>> >>
>> >> I can do this with a macro easily, but is it possible to do it with a
>> >> function?
>
> Don't do it at all. Sonya Keene was wrong. CLOS is very powerful and
> you lose all that power when you hide make-instance behind a maker
> function.

Indeed MAKE-INSTANCE let you use the factory pattern for free!

Compare:

---(factory pattern in some non-descript language)----------------------

/* Factory and car interfaces */

interface CarFactory {
public function makeCar();
}

interface Car {
public function getType();
}

/* Concrete implementations of the factory and car */

class SedanFactory implements CarFactory {
public function makeCar() {
return new Sedan();
}
}

class Sedan implements Car {
public function getType() {
return 'Sedan';
}
}

/* Client */

$factory = new SedanFactory();
$car = $factory->makeCar();
print $car->getType();

------------------------------------------------------------------------

with:

---(factory pattern in lisp)--------------------------------------------

(defclass car ()
((type :initform "generic car" :initarg :type :reader car-type)))

(defclass sedan (car)
()
(:default-initargs :type "sedan"))

(let ((factory 'sedan))
(car-type (make-instance factory)))
; --> "sedan"

------------------------------------------------------------------------

--
__Pascal Bourguignon__ http://www.informat...
â??The factory of the future will have only two employees, a man and a
dog. The man will be there to feed the dog. The dog will be there to
keep the man from touching the equipment.â? -- Carl Bass CEO Autodesk

Kaz Kylheku

7/24/2015 7:13:00 PM

On 2015-07-24, His Kennyness <kentilton@gmail.com> wrote:
> On Saturday, July 18, 2015 at 2:14:07 PM UTC-4, Mart van de Wege wrote:
>> >> I'm trying to build a constructor function that calls make-instance, but
>> >> the number of arguments to pass to make-instance is unknown and depends
>> >> on the number of parameters passed to the original function.
>> >>
>> >> I can do this with a macro easily, but is it possible to do it with a
>> >> function?
>
> Don't do it at all. Sonya Keene was wrong. CLOS is very powerful and you lose
> all that power when you hide make-instance behind a maker function.

Nope! You just have to take the class parameter in the wrapper.

(defun make (class &rest args)
(apply #'make-instance class args))

See? No power lost.

comp.lang.lisp

Passing variable keywords to another function

Mart van de Wege

Pascal J. Bourguignon

Dimitri Fontaine

Pascal Costanza

gengyangcai

Pascal J. Bourguignon

Mart van de Wege

Kenneth Tilton

Mart van de Wege

Pascal J. Bourguignon

Kaz Kylheku

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