Name Rascal/Patterns/Abstract/TypeConstrained

Synopsis Type constrained abstract pattern.

Syntax [Type] Pat

Description A type constrained pattern matches provided that the subject has type Type and Pat matches. This can be handy in case of ambiguity (say more than one constructor with the same name), or in case the pattern is completely general. See an example below:

This does not seem to work properly. There is a bug.

Examples

rascal>import IO;
ok

Some example data type which contains generic values as well as specific expressions:

rascal>data Exp = val(value v) | add(Exp l, Exp r) | sub(Exp l, Exp r);
ok
rascal>ex = add(add(val("hello"(1,2)),val("bye")), sub(val(1),val(2)));
Exp: add(
  add(
    val("hello"(1,2)),
    val("bye")),
  sub(
    val(1),
    val(2)))

Here we constraint the match to find only Exps:

rascal>visit (ex) {
>>>>>>>  case [Exp] str name(_,_) : println("node name is <name>");
>>>>>>>}
node name is hello
node name is add
node name is sub
node name is add
Exp: add(
  add(
    val("hello"(1,2)),
    val("bye")),
  sub(
    val(1),
    val(2)))

Here we do not constrain the same pattern:

rascal>visit (ex) {
>>>>>>>  case str name(_,_) : println("node name is <name>");
>>>>>>>}
node name is hello
node name is add
node name is sub
node name is add
Exp: add(
  add(
    val("hello"(1,2)),
    val("bye")),
  sub(
    val(1),
    val(2)))

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