Name Rascal/Concepts/CaseDistinction

Synopsis Case distinction via pattern matching.

Description The switch statement as known from C and Java is generalized: the subject value to switch on may be an arbitrary value and the cases are arbitrary patterns followed by a statement. Each case is comparable to a transaction: when the pattern succeeds and the following statement is executed successfully, all changes to variables made by the statement are committed and thus become permanent. The variables bound by the pattern are always local to the statement associated with the case. When a match fails or when the associated statement fails, a rollback takes place and all side-effects are undone. External side-effects like I/O and side-effects in user-defined Java code are not undone.

See Switch,Visit and PatternWithAction for more details.

Examples We use the Recipes:ColoredTrees datatype as example and use a switch to distinguish between red and black nodes:

rascal>data ColoredTree = 
>>>>>>>      leaf(int N) 
>>>>>>>    | red(ColoredTree left, ColoredTree right) 
>>>>>>>    | black(ColoredTree left, ColoredTree right);
ok
rascal>ColoredTree CT 
>>>>>>>       = red(black(leaf(1), red(leaf(2),leaf(3))), black(leaf(3), leaf(4)));
ColoredTree: red(
  black(
    leaf(1),
    red(
      leaf(2),
      leaf(3))),
  black(
    leaf(3),
    leaf(4)))
rascal>import IO;
ok
rascal>switch (CT){
>>>>>>>case red(_, _):
>>>>>>>     println("A red root node");
>>>>>>>case black(_, _):
>>>>>>>     black("A black rood node");
>>>>>>>}
A red root node
ok

[Edit] | [New Subconcept] | [Recompile Course] | [Warnings]