How to cast a value type to Map in Rascal?

Question

I have a variable of type value that stores a map, but I can not access the values by providing the keys:

rascal>a
value: ("s":"s")

rascal>a["s"]
|stdin:///|(2,3,<1,2>,<1,5>): subscript not supported on value at |stdin:///|(2,3,<1,2>,<1,5>)
☞ Advice

How can I parse the value to map in order to be able to retrieve my value ?

Answer 1

if (map[str,str] myMap := a) {
   // do stuff with myMap
}
else {
  throw "<a> is not a map?";
}

Another way of "narrowing types" is using pattern matching in function parameters:

rascal>value x = 1;
int: 1
rascal>int myFunc(int i) = 2 * i;
ok
rascal>myFunc(x);
int: 2

And yet another way is using visit or switch:

visit(bigValue) {
  case Expression e => ...work with e...
}

The general idea is:

• pattern matching means narrowing (downcasting)
• pattern matching may fail and so is always in a conditional context
• there are many places in Rascal where you can use pattern matching: function dispatch, switch, visit, :=, <-