In Can Your Programming Language Do This?, Joel Spolsky shows a few examples in JavaScript of using a function as a value. In one example, he demonstrates the use of anonymous functions, which are unnamed and defined right where they are used. He claims that:

...Programming languages with first-class functions let you find more opportunities for abstraction, which means your code is smaller, tighter, more reusable, and more scalable.

While not a purely functional language, the Ruby language does facilitate a functional style of programming, and it has seen a rather significant rise in popularity of late. In the Java world, the Scala language facilitates a functional programming style on the JVM.

To what extent do you think functional programming will become more mainstream in commercial development?

irb(main):008:0> a = lambda {|i| i * 2}
=> #<Proc:0x02ace390@(irb):8>
irb(main):009:0> b = lambda {|i| a.call(i + 2)}
=> #<Proc:0x02aca010@(irb):9>
irb(main):010:0> a.call 3
=> 6
irb(main):011:0> b.call 3
=> 10

class Prok < Proc
  def call(a)
    super(a + 2)
  end
end
 
Proc.new {|a| a * 2}.call(4) => 8
Prok.new {|a| a * 2}.call(4) => 12

class TimesTwo implements SideEffectFree {
   int call(int a) {
      return a * 2;
   }
}
 
// ...
 
new TimesTwo();