$foreach and $exists functions about tosca-community-contributions

[pmbruun]

I would prefer $all or $forall, since $foreach would normally be a distribution function.

This amounts, in the general case, to TOSCA supporting lambdas, and there are two different functionalities that you may want to achieve: distribution and aggregation.

• Both $foreach and $exists are examples of aggregation, using AND and OR respectively. You could just as well want an aggregating SUM.
• Alternatively, you would could want distribution - a function that applies the expression to each element in a list and returns the results in a new list.

Consider now that functions $and, $or, $sum, etc. applied to a list of booleans/numbers automatically aggregate. So @and: <list-expression>* returns the conjunction of the elements of the lists (potentially skipping any null elements?), and @or: <list-expression>* returns the disjunction of the list elements.

Now if we define $each: [ <list of nodes>, <variable name>, <expression> ] as evaluating the expression for each element of the list expanded with the variable name into the expression, then you can express $forall as:

$and:
- $each: 
  - colo-group
  - X
  - $foreach: 
    - colo-group
    - Y
    - $equal:
      - [ X, RELATIONSHIP, host, TARGET, uuid ]
      - [ Y, RELATIONSHIP, host, TARGET, uuid ]

You may of course still have shorthand functions $forall and $exists for this.

[lauwers]

Yes, I believe we want aggregation semantics, not distribution.
By the way, Calin raised a concern about the fact that the proposal above uses the same function syntax for defining function values (i.e. values that are obtained by evaluating a function) and for function arguments ("function pointers") to the $forall and $exists functions. Any opinions on that?