Using an 'is' expression when the right-hand operand is a variable? - dart

Question

I am trying to write a function that takes two arguments: givenType and targetType. If these two arguments match, I want givenType to be returned, otherwise null.
For this objective, I am trying to utilize Dart's is expression (maybe there is a better way to go about it, I am open to suggestions). Initially, I thought it would be as simple as writing this:
matchesTarget(givenType, targetType) {
if (givenType is targetType) {
return givenType;
}
return null;
}
But this produces an error:
The name 'targetType' isn't a type and can't be used in an 'is'
expression. Try correcting the name to match an existing
type.dart(type_test_with_non_type)
I tried looking up what satisfies an is expression but cannot seem to find it in the documentation. It seems like it needs its right-hand operand to be known at compile-time (hoping this is wrong, but it does not seem like I can use a variable), but if so, how else can I achieve the desired effect?

Answer 1

I cant guess the purpose of the function (or the scenario where it would be used, so if you can clarify it would be great). First of all, I don't know if you are passing "types" as arguments. And yes, you need to specify in compile time the right hand argument of the is function.
Meanwhile, if you are passing types, with one change, you can check if the types passed to your function at runtime.
matchesTarget(Type givenType, Type targetType) {
print('${givenType.runtimeType} ${targetType.runtimeType}');
if (givenType == targetType) {
return givenType;
}
return null;
}
main(){
var a = int; //this is a Type
var b = String; //this is also a Type
print(matchesTarget(a,b)); //You are passing different Types, so it will return null
var c = int; //this is also a Type
print(matchesTarget(a,c)); //You are passing same Types, so it will return int
}
But if you are passing variables, the solution is pretty similar:
matchesTarget(givenVar, targetVar) {
print('${givenVar.runtimeType} ${targetVar.runtimeType}');
if (givenVar.runtimeType == targetVar.runtimeType) {
return givenVar.runtimeType;
}
return null;
}
main(){
var a = 10; //this is a variable (int)
var b = "hello"; //this is also a variable (String)
print(matchesTarget(a,b)); //this will return null
var c = 12; //this is also a variable (int)
print(matchesTarget(a,c)); //this will return int
}
The Final Answer
matchesTarget(givenVar, targetType) {
print('${givenVar.runtimeType} ${targetType}');
if (givenVar.runtimeType == targetType) {
return givenVar;
}
return null;
}
main(){
var a = 10; //this is a variable (int)
var b = String; //this is a type (String)
print(matchesTarget(a,b)); //this will return null because 'a' isnt a String
var c = int; //this is also a type (int)
print(matchesTarget(a,c)); //this will return the value of 'a' (10)
}

Answer 2

The as, is, and is! operators are handy for checking types at runtime.
The is operator in Dart can be only used for type checking and not checking if two values are equal.
The result of obj is T is true if obj implements the interface specified by T. For example, obj is Object is always true.
See the below code for an example of how to use the is operator
if (emp is Person) {
// Type check
emp.firstName = 'Bob';
}
Even the error message that you're getting says that
The name 'targetType' isn't a type and can't be used in an 'is'
expression.
So the bottomline is that you can use is only for checking if a variable or value belongs to a particular data type.
For checking equality, you can use the == operator if comparing primitive types, or write your own method for comparing the values. Hope this helps!