= List operations

== List concatenation

Lists can be concatenated using the `++` operator.

[source]
.Example
....
nix-repl> [ 1 2 3 ] ++ [ "apple" "banana" ]
[ 1 2 3 "apple" "banana" ]
....

== Useful built-in functions for lists

Nix provides some built-in functions for working with lists;
a few examples are shown below.
For more information on these and other built-in functions, see the Nix Manual
(https://nixos.org/manual/nix/stable/language/builtins).

Testing if an element appears in a list.

[source]
.Example
....
nix-repl> fruit = [ "apple" "banana" "cantaloupe" ]

nix-repl> builtins.elem "apple" fruit
true

nix-repl> builtins.elem "broccoli" fruit
false
....

Selecting an item from a list by index.
The first element in a list has index `0`.

[source]
.Example
....
nix-repl> builtins.elemAt fruit 0
"apple"

nix-repl> builtins.elemAt fruit 2
"cantaloupe"
....

Determining the number of elements in a list.

[source]
.Example
....
nix-repl> builtins.length fruit
3
....

Accessing the first element of a list.

[source]
.Example
....
nix-repl> builtins.head fruit
"apple"
....

Dropping the first element of a list.

[source]
.Example
....
nix-repl> builtins.tail fruit
[ "banana" "cantaloupe" ]
....

Functions are useful for working with lists.
Functions will be introduced in <<functions>>,
but the following examples should be somewhat self-explanatory.

Using a function to filter (select elements from) a list.

[source]
.Example
....
nix-repl> numbers = [ 1 3 6 8 9 15 25 ]

nix-repl> isBig = n: n > 10                 # is the number "big" (greater than 10)?

nix-repl> builtins.filter isBig numbers     # get just the "big" numbers
[ 15 25 ]
....

Applying a function to all the elements in a list.

[source]
.Example
....
nix-repl> double = n: 2*n                # multiply by two

nix-repl> builtins.map double numbers    # double each element in the list
[ 2 6 12 16 18 30 50 ]
....