`Tuple`

A tuple is a group of elements. To store 2 or more pieces of information, a tuple can be used. Each part can be named and given a type.

With a tuple, we have an immutable collection of elements of any type. All the items can have the same type, but this is not required.

First example

In tuples we often encounter the terms "packing" and unpacking. We pack values into a tuple. Here we place the values "cat," 100 and 4 into a single tuple element.

Then We unpack the tuple's items into 3 variables (animal, weight, feet). We can then address those variables directly.

// Create a tuple with three items.
// ... It has a string and 2 ints.
let data = ("cat", 100, 4)
printfn "%A" data

// Unpack the tuple into 3 variables.
let (animal, weight, feet) = data
printfn "%A" animal
printfn "%A" weight
printfn "%A" feet
("cat", 100, 4)
"cat"
100
4

Multiple return values

With tuples we can return multiple values from a function. Here the testFunction uses a match construct. And it returns tuples.

// This function returns a tuple with the argument and its English word.
// ... It uses a match construct.
let testFunction x =
    match x with
    | 0 -> (0, "zero")
    | 1 -> (1, "one")
    | _ -> (-1, "unknown")

// Test the function with three values.
let result0 = testFunction 0
printfn "%A" result0

let result1 = testFunction 1
printfn "%A" result1

let result2 = testFunction 2
printfn "%A" result2
(0, "zero")
(1, "one")
(-1, "unknown")

Underscore

Sometimes we want to assign a name to one item in a tuple. We can use the underscore "_" to ignore parts of a tuple. These cannot be referenced again.

Here We create a 2-item tuple (a pair) and then set "size" to the first item in the pair. The second item (a string) is ignored.

// This tuple contains two items.
let info = (10, "bird")

// Unpack the tuple, but ignore the second item.
// ... Use an underscore to ignore it.
let (size, _) = info

printfn "%d" size
10

With tuples, we have small, easy-to-declare collections. For more complex objects, a class (declared with the type keyword) is appropriate. Tuples are good for return values.

Tuples are useful

They are a way to group some values of any types together. Unlike a list, the items in a tuple can have different types. We can return tuples from functions.