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Descendant Pattern

rascal-0.34.0

Synopsis​

Deep match in an abstract pattern.

Syntax​

/ Pattern

Description​

A descendant pattern performs a deep match of the pattern Pattern. In other words, it matches when any element of the subject at any depth that matches Pat and is used to match, for instance, tree nodes at an arbitrary distance from the root.

Examples​

rascal>import IO;
ok
rascal>data ColoredTree = leaf(int N)
>>>>>>>                 | red(ColoredTree left, ColoredTree right) 
>>>>>>>                 | black(ColoredTree left, ColoredTree right);
ok
rascal>T = red(red(black(leaf(1), leaf(2)), black(leaf(3), leaf(4))), black(leaf(5), leaf(4)));
ColoredTree: red(
  red(
    black(
      leaf(1),
      leaf(2)),
    black(
      leaf(3),
      leaf(4))),
  black(
    leaf(5),
    leaf(4)))

Now we match for black nodes with leaf(4) as second argument:

rascal>for(/black(_,leaf(4)) := T)
>>>>>>>    println("Match!");
Match!
Match!
list[void]: []

We use an anonymous variable _ at a position where we don't care about the actual value that is matched. In order to print the actual values of the matches, we would need an [Abstract/Labelled] pattern.

Here we match all leaves that occur as second argument of black:

rascal>for(/black(_,leaf(int N)) := T)
>>>>>>>    println("Match <N>");
Match 2
Match 4
Match 4
list[void]: []

Here we list all integers that occur in any leaf:

rascal>for(/int N := T)
>>>>>>>    println("Match <N>");
Match 1
Match 2
Match 3
Match 4
Match 5
Match 4
list[void]: []

Rather than printing, we can also collect them in a list using [$Statements/Append]:

rascal>for(/int N := T)
>>>>>>>    append N;
list[int]: [1,2,3,4,5,4]