Position of a node?

Question

Rascal is rooted in term rewriting. Does it have built-in support for term/node position as commonly defined in term rewriting so that I can query for the position of a sub-term inside a term or the other way around?

Answer 1

I don't believe explicit positions are commonly defined in the semantics of term rewriting, but nevertheless Rascal defines all kinds of operations on terms such that positions are explicit or can be made explicit. Please also have a look at he manuals at http://www.rascal-mpl.org

The main operation on terms is pattern matching using normal first order congruence, deep (higher order) match, negative match, disjunctive match, etc:

if (and(and(_, _), _) := and(and(true(),false()), false())) // pattern match operator :=
  println("yes!"); 

and(true(), b) = b; // function definition, aka rewrite rule
and(false(), _) = false();

[ a | and(a,b) <- booleanList]; // comprehension with pattern as filter on a generator

innermost visit (t) { // explicit automated traversal with strategies
   case and(a,b) => or(a,b)
}

b.leftHandSide = true(); // assign new child term to the leftHandSide field of the term assigned to the b variable (non-destructively, you get a new b)
b[0] = false(); // same but to the anonymous first child.

Then there are the normal projection operators, index on the children term[0] and child-by-name: term.myChildName if there was a many sorted term signature defined using field labels.

if you want to know at which position a sub-child is, I would perhaps write it as such:

int getPos(node t, value child) = [*pre, child, *_] := getChildren(t) ? size(pre) : -1;

but there are other ways of achieving the same.

Rascal does not have pointers to the parents of a term.