What does this '()' notation mean?

f#, function, unit-type, value-type

Solution

Before answering what `()` is lets get some basics defined and some examples done.

In F# a let statement has a name, zero or more arguments, and an expression.

To keep this simple we will go with: If there are no arguments then the let statement is a value. If there are arguments then the let statement is a function.

For a value, the result of the expression is evaluated only once and bound to the identifier; it is immutable. For a function, the expression is evaluated each time the function is called.

So this value

let a = System.DateTime.Now;;

will always have the time when it is first evaluated or later invoked, i.e.

a;;
val it : System.DateTime = 1/10/2017 8:16:16 AM ...  
a;;
val it : System.DateTime = 1/10/2017 8:16:16 AM ...  
a;;
val it : System.DateTime = 1/10/2017 8:16:16 AM ...

and this function

let b () = System.DateTime.Now;;

will always have a new time each time it is evaluated, i.e.

b ();;
val it : System.DateTime = 1/10/2017 8:18:41 AM ...  
b ();;
val it : System.DateTime = 1/10/2017 8:18:49 AM ...  
b ();;
val it : System.DateTime = 1/10/2017 8:20:32 AM ...

Now to explain what `()` means. Notice that `System.DateTime.Now` needs no arguments to work.

How do we create a function when the expression needs no arguments?

Every argument has to have a type, so F# has the unit type for functions that need no arguments and the only value for the unit type is `()`.

So this is a function with one argument `x` of type `int`

let c x = x + 1;;

and this is a function with one argument `()` of type `unit`

let b () = System.DateTime.Now;;

Problem

I just started to learn F#. The book uses the following notation: ``` let name() = 3 name() ``` what that differs from this: ``` let name = 3 name ``` ?

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