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Labelled Pattern

rascal-0.34.0

Synopsis​

Labelled abstract pattern.

Syntax​

Var : Pat

Description​

A labelled pattern matches the same values as Pat, but has as side-effect that the matched value is assigned to Var. Note that there is also Typed Labelled with very similar syntax and semantics.

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)))
rascal>for(/M:black(_,leaf(4)) := T)
>>>>>>>    println("Match <M>");
Match black(leaf(3),leaf(4))
Match black(leaf(5),leaf(4))
list[void]: []

We use an anonymous variable _ at a position where we don't care about the actual value that is matched.