Discussion of Common Lisp
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sabra.crolleton
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by sabra.crolleton » Tue Aug 06, 2013 6:20 am
I'm feeling particularly stupid today. Have a method which specifies on numbers and arrays. When applied to accessors of an clos object, it works on arrays, but not on numbers. The question is why. I would think that passing by value would mean both fail. I would think that passing by reference, both would work. I'm missing something fundamental here.
Code: Select all
(defclass t3 ()
((a :accessor a :initarg :a :initform 1)
(b :accessor b :initarg :b :initform (make-array 5 :initial-element 0))))
(defmethod add-something ((x number) y) (setf x (+ x y)))
(defmethod add-something ((x array) y) (setf (aref x 0) (+ (aref x 0) y)))
(let ((z (make-instance 't3)))
(describe z)
(add-something (a z) 21)
(add-something (b z) 22)
(describe z))
#<T3 {1008FAEC43}>
[standard-object]
Slots with :INSTANCE allocation:
A = 1
B = #(0 0 0 0 0)
#<T3 {1008FAEC43}>
[standard-object]
Slots with :INSTANCE allocation:
A = 1
B = #(22 0 0 0 0)
Question: why does (b z) get set but (a z) does not get set?
Last edited by
sabra.crolleton on Tue Aug 06, 2013 8:23 am, edited 1 time in total.
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Kompottkin
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by Kompottkin » Tue Aug 06, 2013 6:54 am
Lisp is call-by-value, not call-by-reference. In
Code: Select all
(defmethod add-something ((x number) y) (setf x (+ x y)))
the variable
x is local to the method. When the method is called, (a pointer to) the argument object is
copied into the
x variable. There is no way for
add-something to modify whatever the place the supplied value originates from is.
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Kompottkin
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by Kompottkin » Tue Aug 06, 2013 1:38 pm
Because Lisp always passes references to objects (by value). Therefore, you can modify objects passed to you, but not the references to those objects. Your array method modifies the passed vector, which is an effect that is visible to the caller. Your number method modifies a locally bound reference to the passed number, not the number itself (which cannot be modified at all).
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Goheeca
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by Goheeca » Wed Aug 07, 2013 7:57 am
It looks like a bad approach. Why want you to do that? The special expressions
places are different from a variable therefore this behaviour. Maybe I'll come up with a solution based on
references, it needs create a reference datatype based on a referee, not an ultimate one.
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sabra.crolleton
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by sabra.crolleton » Wed Aug 07, 2013 10:22 pm
Why was I asking? I am running some simulation models with different degrees of analysis. In the most simplistic model, the clos slots are merely ratios. Each more detailed level of analysis adds another dimension of data. Thus the slots of the clos objects in level 2 contain vectors, the slots in level 3 contain 2 dimensional arrays, the slots in level 4 contain 3 dimensional arrays. Generic methods allow me to call the same method names, but the data manipulation would be appropriate for the different levels of analysis. I actually started with levels 2-4, where everything worked,then was asked to create an introductory level without really thinking that the same process would not work if the slot only contained a number and not an object. The example posted was the simplest example I could create to show my lack of understanding.
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Kompottkin
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by Kompottkin » Thu Aug 08, 2013 5:55 am
sabra.crolleton wrote:Each more detailed level of analysis adds another dimension of data. Thus the slots of the clos objects in level 2 contain vectors, the slots in level 3 contain 2 dimensional arrays, the slots in level 4 contain 3 dimensional arrays.
So the first level ought to contain 0-dimensional arrays.
Code: Select all
(let ((a (make-array '())))
(setf (aref a) 100)
(incf (aref a))
(aref a))
;=> 101
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Goheeca
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by Goheeca » Thu Aug 08, 2013 6:14 am
Yeah, from a mathematical point of view the 0D array is a nice solution.
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sylwester
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by sylwester » Thu Aug 08, 2013 1:51 pm
Wouldn't it be better to make a struct? Perhaps like Scheme SRFI-111 box?
I'm the author of
two useless languages that uses
BF as target machine.
Currently I'm planning a Scheme compiler :p
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sabra.crolleton
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by sabra.crolleton » Fri Aug 09, 2013 4:30 pm
I like the 0 dimensional array idea. It makes everything mathematically consistent in my mind.
Thank you.