You can access the value of an expression inside a string by using ${expression}.

var greeting = "Hello";
var person = "Rohan";

print("${greeting}, ${person}!"); // prints "Hello, Rohan!"

If the expression is an identifier, the {} can be skipped.

print("$greeting, $person");

If the variable inside the {} isn’t a string, the variable’s toString() method is called:

int x = 5;
print("There are ${x.toString()} people in this room");

The call to toString() is unnecessary (although harmless) in this case: toString() is already defined for ints and Dart automatically calls toString(). What this does mean, though, is that it is the user’s responsibility to define a toString() method when interpolating user-defined objects.

You can interpolate expressions of arbitrary complexity by placing them inside ${}:

A ternary if..else:

int x = 5;
print("There are ${x < 10 ? "a few" : "many"} people in this room");

List and Map operations:

List list = [1, 2, 3, 4, 5];
print("The list is $list, and when squared it is ${list.map((i) {return i * i;})}");
// The list is [1, 2, 3, 4, 5], and when squared it is [1, 4, 9, 16, 25]

Map map = {"ca": "California", "pa": "Pennsylvania"};
print("I live in sunny ${map['ca']}");
// I live in sunny California

Notice above that you can access $list(an identifier) without using {}, but the call to list.map(an expression) needs to be inside {}. Similarly, in the example below, $x works, but {} are required for -x:

print("x = $x and -x = ${-x}");
// x = 5 and -x = -5

Expressions inside ${} can be arbitrarily complex:

List names = ['John', 'Sophena', 'Henrietta'];
print("${
  ((names) {
      return names[(new math.Random()).nextInt(names.length)];
  })(names)} is the most valueable member of our team");

The code above defines an anonymous function to pick a random name from a list and then calls that function with names as an argument. All of this is done as part of string interpolation.

Creating a function and immediately calling it is useful in a lot of situations (it is a common practice in Javascript); but, watch out: doing this sort of thing can lead to hard to maintain code. An abudance of caution is advised ;)