Types and datatypes

[doganulus]

I would like to share my observations about VSS types and datatypes. The current classification is as follows:

type: [sensor, actuator, attribute, property, struct, branch]
datatype: [boolean, string, numbers, arrays]

I have two questions here:

• First, the difference between types attribute and property is unclear to me. Let me try to summarize my understanding below. Is that right?

  • sensor denotes a dynamic (temporal) signal not directly controllable by the system -- fast change
  • actuator denotes a dynamic (temporal) signal directly controllable by the system -- fast change
  • attribute denotes a dynamic? configuration variable -- thus, runtime-changeable but not often.
  • property denotes a static? configuration parameter? -- runtime-constant

• Second, I understand that struct is a relatively new keyword for the VSS project. My initial assessment would place the structural elements of struct and branch under datatype as I could not find a functional description for struct as above. The same is true for branch where the difference is that structs are packed, and branches are not. Therefore, what would say about the following classification that moves them under datatypes?

type: [sensor, actuator, attribute, property]
datatype: [boolean, string, numbers, arrays, struct, branch]

Even more, taking full liberty to ignore all necessary backward compatibility issues (probably the project cannot do very easily), I would prefer the key type instead of datatype for compatibility with other specification formats, mainly JSON Schema, and prefer another term, like kind or class, to denote its usage in the system.

kind: [sensor, actuator, attribute, property]
type: [null, boolean, string, numbers, tuple, list, struct, branch]

And, a straightforward mapping to JSON types may be valuable as I would map tuple and list to JSON arrays as well as struct and branch to JSON objects. But I admit I don't have much prior experience with other formats that VSS can be converted.

[jsyvanen]

I understand that struct is a relatively new keyword for the VSS project. My initial assessment would place the structural elements of struct and branch under datatype as I could not find a functional description for struct

I was also perplexed by this since the current docs for struct seem to use type and datatype interchangeably in a few places. But I believe the intention is that struct is a datatype and not a type in the sense of sensor, actuator, attribute.

the difference between types attribute and property is unclear to me

Relating to the above definition of struct as a datatype, my best interpretation at the moment is that property is a "type" intended to be used only inside struct definitions. Indeed it is the only place where the property "type" is referenced within the repo at the moment.

It would be great to have the understanding confirmed/disconfirmed. If the above is correct I would like to understand the intended usage of the property "type" in the context of struct datatypes.

[jsyvanen]

@erikbosch thanks for #688 . Would you be able to comment on the specific question above about intended use of property ?

[erikbosch]

@jsyvanen - you can check som struct examples in vss-tools for testing, like the one in https://github.com/COVESA/vss-tools/blob/master/tests/vspec/test_structs/TestBranch1.vspec

As of today type: property is intended to be used only to specify members of a struct type. So the snippet below

NestedStruct:
  type: struct
  description: "A struct that is going to be used within another struct - Nested"

NestedStruct.x:
  type: property
  description: "x property"
  datatype: double
  min: -10

NestedStruct.y:
  type: property
  description: "y property"
  datatype: double
  max: 10

NestedStruct.z:
  type: property
  description: "z property"
  datatype: double
  default: 1

Would if represented in for example C++ be something like

struct NestedStruct {
  double x;
  double y;
  double z;
};

I.e. the VSS properties correspond to C++ struct members.