When to use num in Dart

One benefit for example before Dart 2.1 :

suppose you need to define a double var like,

double x ;

if you define your x to be a double, when you assign it to its value, you have to specify it say for example 9.876.

x = 9.876;

so far so good.

Now you need to assign it a value like say 9

you can't code it like this

x = 9;  //this will be error before dart 2.1

so you need to code it like

x = 9.0;

but if you define x as num

you can use

x = 9.0;

and

x = 9;

so it is a convenient way to avoid these type mismatch errors between integer and double types in dart.

both will be valid.

this was a situation before Dart 2.1 but still can help explain the concept

check this may be related

Not sure if this is useful to anyone, but I just ran into a case where I needed num in a way.

I defined a utility function like this:

T maximumByOrNull<T, K extends Comparable<K>>(Iterable<T> it, K key(T),) {
  return it.isEmpty
      ? null
      : it.reduce((a, b) => key(a).compareTo(key(b)) > 0 ? a : b);
}

And invoking it like this…

eldest = maximumByOrNull(students, (s) => s.age);

… caused trouble when age is an int, because int itself does not implement Comparable<int>.

So Dart cannot infer the type K in the invocation to maximumByOrNull.

However, num does implement Comparable<num>. And if I specified:

eldest = maximumByOrNull(students, (s) => s.age as num);         // this
eldest = maximumByOrNull<Student, num>(students, (s) => s.age);  // or this

the complaint went away.

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Bottom line: it seems num implements Comparable when int and double do not, themselves, and sometimes this causes trouble for generic functions.