Why doesn't this work
Type type = double;
switch (type) {
case int:
print('int');
case double:
print('double');
}
But this does
switch (type) {
case const (int):
print('int');
case const (double):
print('double');
}
What is const (Type) for? It looks like turning a class or type (hereinafter simply class) into its constant form. Well, firstly, turning something into a constant seems wrong to me, like, we just define them and forbid redefining them. What's the point of making something immutable only for a certain period, instead of just not changing it? Secondly, isn't the class itself immutable during runtime, which in essence can be perceived as a constant?
Why not just use the first form to write switch-case for comparing types? Either I don't understand what const (Type) does, or I don't understand what a class is (if so, then I hope only in Dart).
The code works.
The question might be promted by lint warning about not using case int:.
It's discouraged because people have mistakenly written switch (someValue) { case int: ...} and expected it to check whether the value is an integer.
Instead it checks if the value is a Type object equal to int, with no warning about it not being able to match (because it could hypothetically be an instance of a sub class of the switched-on type that implements Type).
You should use int _ or int() to check whether the value is an integer, and to avoid the lint warning, you can use == int or const (int) to emphasize that you really do want to compare to a Type object.
The const (expression) is a way to put any constant expression into a pattern. Writing const (int) doesn't mean anything other than int, it's only there to silence the lint by telling it that you know you're comparing to a constant Type object.
Generally, comparing Type objects is usually a sign of a class design that doesn't match the problem. It's also usually fragile and error prone code.