F# The "unit" type What good is a 0-tuple?
Example
A 2-tuple or a 3-tuple represent a group of related items. (Points in 2D space, RGB values of a color, etc.) A 1-tuple is not very useful since it could easily be replaced with a single int.
A 0-tuple seems even more useless since it contains absolutely nothing. Yet it has properties that make it very useful in functional languages like F#. For example, the 0-tuple type has exactly one value, usually represented as (). All 0-tuples have this value so it's essentially a singleton type. In most functional programming languages, including F#, this is called the unit type.
Functions that return void in C# will return the unit type in F#:
let printResult = printfn "Hello"
Run that in the F# interactive interpreter, and you'll see:
val printResult : unit = ()
This means that the value printResult is of type unit, and has the value () (the empty tuple, the one and only value of the unit type).
Functions can take the unit type as a parameter, too. In F#, functions may look like they're taking no parameters. But in fact, they're taking a single parameter of type unit. This function:
let doMath() = 2 + 4
is actually equivalent to:
let doMath () = 2 + 4
That is, a function that takes one parameter of type unit and returns the int value 6. If you look at the type signature that the F# interactive interpreter prints when you define this function, you'll see:
val doMath : unit -> int
The fact that all functions will take at least one parameter and return a value, even if that value is sometimes a "useless" value like (), means that function composition is a lot easier in F# than in languages that don't have the unit type. But that's a more advanced subject which we'll get to later on. For now, just remember that when you see unit in a function signature, or () in a function's parameters, that's the 0-tuple type that serves as the way to say "This function takes, or returns, no meaningful values."
