How to cast data of type 'value' to other type of values in Rascal?

Question

I have a variable x = value: |java+interface:///java/util/Iterator|. How to convert this to x = loc: |java+interface:///java/util/Iterator|?

Answer 1

Simple answer, pattern matching is like casting. You can use it in different ways to make a static type more concrete:

value x = ... ;
if (loc l := x) {
   // here l will have the type `loc` and have the same value that `x` had
}

or:

value x = ...;
bool myFunc(loc l) = ...  ;
myFunc(x); // /* will only be executed if  x is indeeed of type `loc`

or, you can use pattern matching to filter a list:

list[value] l = ...;
list[loc] ll = [ e | loc e <- l ];

or, a switch:

switch(x) {
  case loc l: ...;
}

etc. :-)

A generic cast function can also be written, but I think it's a code smell to use it and it makes code harder to understand:

&T cast(type[&T] t, value x) {
  if (&T e := x) 
     return e;
  throw "cast exception <x> can not be matched to <t>";
} 

value x = ...;
loc l = cast(#loc, x);