PutLink can give different results in case GetLink or result of it executin is used

[stellarspot]

Run the example below where ListLink uses GetLinks inside:

(use-modules (opencog) (opencog exec))

(Inheritance (Concept "Jim") (Concept "Father"))
(Inheritance (Concept "Jane") (Concept "Mother"))

(define put-get-multi
 (PutLink
  (LambdaLink
   (VariableList (Variable "x") (Variable "y"))
   (EvaluationLink
    (PredicateNode "relatives")
    (ListLink
     (Variable "x")
     (Variable "y")
     (Concept "mom and pop"))))
  (ListLink
   (GetLink (TypedVariable (Variable "$X") (Type 'ConceptNode))
    (Inheritance (Variable "$X") (Concept "Father")))
   (GetLink (TypedVariable (Variable "$X") (Type 'ConceptNode))
    (Inheritance (Variable "$X") (Concept "Mother"))))))


(display
 (cog-execute! put-get-multi))

The result is:

(EvaluationLink
   (PredicateNode "relatives")
   (ListLink
      (ConceptNode "Jim")
      (ConceptNode "Jane")
      (ConceptNode "mom and pop")
   )
)

Here is the updated example where GetLinks are executed and the result is used inside of ListLink:

(use-modules (opencog) (opencog exec))

(Inheritance (Concept "Jim") (Concept "Father"))
(Inheritance (Concept "Jane") (Concept "Mother"))

(define father
 (cog-execute!
  (GetLink (TypedVariable (Variable "$X") (Type 'ConceptNode))
   (Inheritance (Variable "$X") (Concept "Father")))))

(define mother
 (cog-execute!
  (GetLink (TypedVariable (Variable "$X") (Type 'ConceptNode))
   (Inheritance (Variable "$X") (Concept "Mother")))))

(define put-get-multi
 (PutLink
  (LambdaLink
   (VariableList (Variable "x") (Variable "y"))
   (EvaluationLink
    (PredicateNode "relatives")
    (ListLink
     (Variable "x")
     (Variable "y")
     (Concept "mom and pop"))))
  (ListLink
    father
    mother)))

(display
 (cog-execute! put-get-multi))

The result is:

(EvaluationLink
   (PredicateNode "relatives")
   (ListLink
      (SetLink
         (ConceptNode "Jim")
      )
      (SetLink
         (ConceptNode "Jane")
      )
      (ConceptNode "mom and pop")
   )
)

Now SetLink is used inside the result.

There should be explicit documentation that needs to properly clarify what is a result of LambdaLink execution when its arguments are SetLink.

(PutLink
 (LambdaLink
  (VariableList (Variable "X"))
  ...
 )
 (ListLink
  (SetLink Y1 Y2 Y3)))

a) LambdaLink is executed only once and X is substituted by (SetLink Y1 Y2 Y3)
b) LambdaLink is executed 3 times where X is substituted by Y1, Y2, Y3
c) LambdaLink is executed only once with the first argument of the SetLink where X is substituted by Y1
d) LambdaLink is executed only once with the random value from the SetLink

[stellarspot]

I run the code where the result of GetLink is empty:

(Inheritance (Concept "Jane") (Concept "Mother"))

(define put-get-multi
 (PutLink
  (LambdaLink
   (VariableList (Variable "x") (Variable "y"))
   (EvaluationLink
    (PredicateNode "relatives")
    (ListLink
     (Variable "x")
     (Variable "y")
     (Concept "mom and pop"))))
  (ListLink
   (GetLink (TypedVariable (Variable "$X") (Type 'ConceptNode))
    (Inheritance (Variable "$X") (Concept "Father")))
   (GetLink (TypedVariable (Variable "$X") (Type 'ConceptNode))
    (Inheritance (Variable "$X") (Concept "Mother"))))))

(display
 (cog-execute! put-get-multi))

The result contains SetLink:

(EvaluationLink
   (PredicateNode "relatives")
   (ListLink
      (SetLink
      )
      (ConceptNode "Jane")
      (ConceptNode "mom and pop")
   )
)

It looks like if ListLink contains SetLink the LambdaLink needs to be applied to each value inside link (version b).

Anyway, it should be explicitly described in the specification. See #2308

[linas]

Part of what you are describing are bugs in the code, The reason there are bugs in the code is issue #1502 -- It turns out that SetLink was a fundamentally bad idea, precisely because it creates problems like this. There are two paths: a short-term fix: fix the code so that SetLinks are always unwrapped. The long-term fix is to stop using SetLink. The long-term fix is ... does not yet have a good design proposal; it's still in the idea stage.

[linas]

I'm going to close this, because it is fixed in #2310

I'll say it again: I really don't like all the fiddling around with SetLink that is done inside of the PutLink. But in order for it to work "naturally" and "easily" with a mixture of Get and Put, there seems to be no other choice, other than to somehow solve #1502

Hopefully, your examples also show why there has been o forward progress on #1502 -- there are just enough special cases that a good, easy, elegant solution for #1502 is not entirely obvious. Again -- the goal is to have things work "naturally", and be "easy" to use.