Get list of references to an AST node

[MeAmAnUsername]

Short description Some way to get the list of nodes referencing an AST node.

Problem description. I want to find unused variables, imports, etc.

Describe the solution you'd like A built-in function (or a library function) that takes an AST node and returns a list of AST nodes: getRefs : astId -> list(astId). Note: I use astId as general AST node, but if there is another way that would work too. Problems with this solution: the returned list is just general AST nodes. Those cannot be used in function that expect a specific sort without casting, which is currently not implemented.

Describe alternatives you've considered 1 To solve the problem, a function to just return whether a node has references would be enough:isUsed : astId -> BOOLEAN. BOOLEAN is not a built-in type though. Additionally, returning a list of references may be useful for purposes besides my current use-case.

2 Detecting unused variables can (probably) also be done with a helper relation where a declaration is made when a variable is used, which can then be checked by the original node. I think this would work without deadlocking, but it seems a little boilerplate-heavy.

relations
  used : string // the name of the variable. Uses string so that it works for every possible name.

rules
  declareVar : scope * VAR_ID * TYPE
  declareVar(s, name, T) :-
    !var[name, T] in s,
    // duplicate checks, etc. omitted for simplicity
    query used in s |-> [_|_] | warning $[variable [name] is unused] @name.

  resolveVar : scope * VAR_ID -> TYPE
  resolveVar(s, name) = T :-
    query var filter {name'' :- name == name'' } in s |-> [(path, (name', T))],
    // could not resolve errors etc. omitted for simplicity
    @name.ref := name',
    !used[name'] in getResolvedScope(path).

  getResolvedScope : path -> scope // get the end scope from a path.

3 Use DynSem to detect and give warnings for unused variables. DynSem can give warnings on useless assignments, so that will almost certainly also work for unused variables. I don't think that this is part of the DynSem domain though.

Additional context Giving warnings on unused variables allows the user to remove those variables, which improves the code quality.

[AZWN]

Thanks for reporting this issue. Note however that a fourth (pretty neat IMHO) alternative exists, which does not require any adaption of Statix, namely defining resolveVar as follows:

  resolveVar(s, name) = T :-
    query var filter {name'' :- name == name'' } in s |-> [(path, (name', T))],
    @name.ref := name',
    @name'.refs += name.

Now, each declaration that has no values in its refs bag property (or only a single one, if resolveVar is used for duplication checking) is effectively unused.

Now, using stx-get-ast-property, this property can be checked for each declaration, and, if needed, appropriate warnings can be generated.

Can you comment on whether this approach is satisfactory for your original problem?

[MeAmAnUsername]

That might work, I'd have to figure out how to generate warnings with Stratego. A cursory glance of the documentation makes it seem like it is not possible to use both Stratego and Statix to generate warnings.

[AZWN]

Ok, as I explained on Slack, I think that is possible. I'm closing this now, but feel free to reopen if it does not work out for some reason.