Entities should record where they're defined.

[jimsynz]

It'd be awesome if all entities contained a record of the line and column numbers for where they're defined in the user's code, something like:

%MyEntity{
  span: %Spark.Span{start: %Spark.Pos{line: 1, col: 1}, end: %Spark.Pos{line: 1, col: 54}},
  # ...
}

This way we could provide better error messages to our users.

[zachdaniel]

Yes I would love this :)