Rascal MPL remove subtree in visit

Question

How to replace a subtree with nothing in a visit?

In Pattern With Action, we see that

case red(l, r) => green(l, r)

replaces all red nodes with green nodes. I want to do something like

case red(l, r) => []

to remove all red nodes. Is that possible?

Answer 1

Interesting question. So if the abstract grammar is this:

data MyNode
   = green(MyNode l, MyNode r)
   | red(MyNode l, MyNode r)
   | black()
   | white()
   ;

Any visit is not allowed to change the type of the node it replaces. So a red node can become green, or black or white, but not "nothing". You could add a definition of nothing:

data MyNode = nothing();

And then rewrite all red nodes to nothing:

case red(_,_) => nothing()

So that's not really nothing :-)

However, a more common case is to remove elements from a list. In abstract syntax trees for programming languages this often happens:

data MyNode = nodes(list[MyNode] elements);

For example if the list of statements of an if-then-else statement, etc.

Now you can match on the list to remove elements:

case nodes([*prefix, red(_,_), *postfix)]) => nodes([*prefix, *postfix])

And now the red node is really gone.