Proposal: magic words as shortcuts for function calls with no arguments

[tliron]

In today's ad hoc discussion we came up with the following:

We like the idea of supporting "magic words" generally in TOSCA as a friendly syntactical shortcut

A "magic word" is a string that is not a YAML map key beginning with "$" (unescaped). It is equivalent to a function call with no arguments (or an empty seq).

For strings that are map keys beginning with "$" (unescaped) the same rules apply as we have previously defined them: we require that the map have only one key (the function name), otherwise it would be a syntax error, and that key's value is the argument or seq of arguments.

In the following example $magic is a function with no arguments that returns an integer:

node_types:

  MyNode1:
    properties:
      prop1:
        type: integer
      prop2:
        type: list
        entry_schema: integer
      prop3:
        type: map
        entry_schema: integer

service_template:

  node_templates:

    my-node:
      type: MyNode1
      properties:
        prop1: $magic
        prop2: [ $magic, $magic, $magic ]
        prop3:
          key1: $magic
          key2: $magic

If we want to use a function call with no arguments as YAML keys, we have to use the full syntax instead of magic words. Here $keygen returns a GUID string:

    my-node:
      type: MyNode2
      properties:
        hints:
          { $keygen: [] }: 123

(There is a potential problem in the example above in situations with multiple identical keys in YAML. See issue #134)

[lauwers]

This proposal has been accepted and is documented in Section 5.4.14 of https://docs.oasis-open.org/tosca/TOSCA/v2.0/csd05/TOSCA-v2.0-csd05.html