Type Constrained Pattern

rascal-0.34.0

Synopsis

Type constrained abstract pattern.

Syntax

[Type] Pattern

Description

A type constrained pattern matches provided that the subject has type Type and Pattern 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:

There are special cases in which the type constrained pattern calls a parser or validator for the given type:

• if the subject is of type str and the Type is a syntax non-terminal the string is parsed using the non-terminal
• if the subject is of type loc and the Type is a syntax non-terminal the contents of the resource that the subject location is referring to is parsed using the non-terminal

danger

Warning: 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 constrain 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)))