writing a function to do type cast

Question

I'm trying to write a function that does type casting, which seems to be a frequently occurring activity in Rascal code. But I can't seem to get it right. The following and several variations on it fail.

public &T cast(type[&T] tp, value v) throws str {
    if (tp tv := v)
        return tv;
    else
        throw "cast failed";
}

Can someone help me out?

Some more info: I frequently use pattern matching against a pattern of the form "Type Var" (i.e. against a variable declaration) in order to tell Rascal that an expression has a certain type, e.g.

map[str,value] m := myexp

This is usually in cases where I know that myexp has type map[str,value], but omitting the matching would make Rascal's type checking mechanism complain. In order to be a bit more defensive against mistakes, I usually wrap the matching construct in an if-then-else where an exception is raised if the match fails:

if (map[str,value] m := myexp) {
  // use m
} else {
  throw "cast failed";
}

I would like to shorten all such similar pieces of code using a single function that does the job generically, so that I can write instead

cast(#map[str,value], myexp)

PS. Also see How to cast a value type to Map in Rascal?

Answer 1

The code has a bug. This is the fixed code:

public &T cast(type[&T] tp, value v) throws str {
    if (&T tv := v)
        return tv;
    else
        throw "cast failed";
}

Note that we do not wish to include this in the standard library. Rather lets collect cases where we need it and find out how to fix it in another way.

If you find you need this casting often, then you might be avoiding the better parts of Rascal, such as pattern based dispatch. See also the answer by Mark Hills.