Annotating a ADT/Node tree with "parent" labels in Rascal - adt

Question

I want to create a tree (using a Node or ADT) in which every node has an annotation pointing back to its parent. Below is an example with a simple linked list data structure:
import util::Math;
import IO;
import Node;
anno LinkedList LinkedList#parent;
anno int LinkedList#something;
data LinkedList = item(int val, LinkedList next)
| last(int val)
;
public LinkedList linkedList = item(5,
item(4,
item(3,
item(2,
last(1)[#something=99]
)[#something=99]
)[#something=99]
)[#something=99]
)[#something=99];
public LinkedList addParentAnnotations(LinkedList n) {
return top-down visit (n) {
case LinkedList x: {
/* go through all children whose type is LinkedList */
for (LinkedList cx <- getChildren(x), LinkedList ll := cx) {
/* setting the annotation like this doesn't seem to work */
cx#parent = x;
// DOESN'T WORK EITHER: setAnnotation(cx, getAnnotations(cx) + ("parent": x));
}
}
}
}
Executing addParentAnnotations(linkedList) yields the following result:
rascal>addParentAnnotations(linkedList);
LinkedList: item(
5,
item(
4,
item(
3,
item(
2,
last(1)[
#something=99
])[
#something=99
])[
#something=99
])[
#something=99
])[
#something=99
]

Answer 1

The thing is that Rascal data is immutable, so you can not update anything using an assignment. The assignment will simply give you a new binding for cx with the annotation set, but will not change the original tree.
To change the original tree, you can either use the => operator for case statements, or the insert statement as follows:
case LinkedList x => x[#parent=...] // replace x by a new x that is annotated
or:
case LinkedList x : {
...
x#parent= ...;
insert x; // replace original x by new x in tree
}
Some other tip, in the Traversal.rsc library you can find a function called getTraversalContext() which produces a list of parents of the currently visited node if called from the body of a case:
import Traversal;
visit (...) {
case somePattern: {
parents = getTraversalContext();
parent = parents[1];
}
}