Values

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### Values

Values are functions! Examples:

    val(3)
    val("Hello, World")

The first evaluates to a nullary function (a function taking no arguments) that returns an `int`, `3`. The second evaluates to a nullary function that returns a `char const(&)[13]`, `"Hello, World"`.

##### Lazy Evaluation

Confused? `val` is a unary function and `val(3)` invokes it, you say? Yes. However, read carefully: "evaluates to a nullary function". `val(3)` evaluates to (returns) a nullary function. Aha! `val(3)` returns a function! So, since `val(3)` returns a function, you can invoke it. Example:

    std::cout << val(3)() << std::endl;

(See values.cpp)

The second function call (the one with no arguments) calls the nullary function which then returns `3`. The need for a second function call is the reason why the function is said to be Lazily Evaluated. The first call doesn't do anything. You need a second call to finally evaluate the thing. The first call lazily evaluates the function; i.e. doesn't do anything and defers the evaluation for later.

##### Callbacks

It may not be immediately apparent how lazy evaluation can be useful by just looking at the example above. Putting the first and second function call in a single line is really not very useful. However, thinking of `val(3)` as a callback function (and in most cases they are actually used that way), will make it clear. Example:

    template <typename F>
    void print(F f)
    {
        cout << f() << endl;
    }

    int
    main()
    {
        print(val(3));
        print(val("Hello World"));
        return 0;
    }

(See callback.cpp)