count until predicate returns false

[Andrej Mitrovic]

This will print the count of whitespace chars in a line:

writeln(count!isWhite(line));

What I need is the count of whitspace chars until the first
non-whitespace char is found, essentially I need a "countWhile"
template:

writeln(countWhile!isWhite(line));

Can I do this with existing templates somehow?