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Unexpected Skill behaviour with disembodied property lists

FormerMember over 4 years ago

Hello,

I encountered a strange behaviour with disembodied property lists, which I do not understand. When I load the following script

procedure( proc1()
   let((x)
      x = '( nil ) ; empty disembodied property list
      printf("Value should be '(nil)' and is '%A'\n" x)
      x->prop = 0
      printf("Value should be '(nil 'prop 0)' and is '%A'\n" x)
   )
)

and call the procedure twice with 'proc1() proc1()' in the CIW, I get the following output:

Value should be '(nil)' and is '(nil)'
Value should be '(nil 'prop 0)' and is '(nil prop 0)'
Value should be '(nil)' and is '(nil prop 0)' <-- x has a wrong value
Value should be '(nil 'prop 0)' and is '(nil prop 0)'

After the first call of 'proc1' the values of 'x' are as expected, but in every subsequent call 'x' still has the same value, even though it is in a local let scope and initialized to '(nil). When I replace "x = '( nil )" with "x = ncons(nil)" in the procedure, it behaves as expected.

Value should be '(nil)' and is '(nil)'
Value should be '(nil 'prop 0)' and is '(nil prop 0)'
Value should be '(nil)' and is '(nil)'
Value should be '(nil 'prop 0)' and is '(nil prop 0)'

Can someone explain this behaviour or what I am doing wrong? I thought a disembodied property list can be initialized as a non-empty list with a single element (usually 'nil')?

Best regards
Paul

Parents

AaronSymko over 4 years ago

When a static list or DPL is used in a function, it is scoped globally. Thus any destructive operation (e.g. x->prop = 0) will apply to the same, global object. The solution is to use the list() function instead of '(). The list() function will construct a new list every time. For example:

procedure( proc1()
let((x)
x = list( nil ) ; empty disembodied property list

...

)

)
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FormerMember over 4 years ago in reply to AaronSymko

Thanks Aaron for the answer. I guessed something like that, but could not reproduce it in the CIW. When I execute the following

x = '(nil)
y = '(nil)
eq(x y) -> nil

the lists are different objects. Is this behaviour somewhere documented? I could not find it in the Skill manual.

Best regards
Paul
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AaronSymko over 4 years ago in reply to FormerMember

The reason why it works in your experiment is because you have separate invocations of '(nil) and so they are different from each other. When called within a function, they are the same literal list. There are a couple of relevant articles which cover this (both written by Andrew Beckett):

Why does my SKILL function remember previous local disembodied property lists?

Copies of disembodied property lists still have links to each other
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AaronSymko over 4 years ago in reply to FormerMember

The reason why it works in your experiment is because you have separate invocations of '(nil) and so they are different from each other. When called within a function, they are the same literal list. There are a couple of relevant articles which cover this (both written by Andrew Beckett):

Why does my SKILL function remember previous local disembodied property lists?

Copies of disembodied property lists still have links to each other
Cancel
Vote Up 0 Vote Down

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