mirror of
https://codeberg.org/mhwombat/nix-book.git
synced 2025-12-26 16:24:56 +08:00
147 lines
2.6 KiB
Text
147 lines
2.6 KiB
Text
= Attribute set operations
|
|
|
|
== Selection
|
|
|
|
The `.` operator selects an attribute from a set.
|
|
|
|
[source]
|
|
.Example
|
|
....
|
|
nix-repl> animal = { name = { first = "Professor"; last = "Paws"; }; age = 10; species = "cat"; }
|
|
|
|
nix-repl> animal . age
|
|
10
|
|
|
|
nix-repl> animal . name . first
|
|
"Professor"
|
|
....
|
|
|
|
== Query
|
|
|
|
We can use the `?` operator to find out if a set has a particular attribute.
|
|
|
|
[source]
|
|
.Example
|
|
....
|
|
nix-repl> animal ? species
|
|
true
|
|
|
|
nix-repl> animal ? bicycle
|
|
false
|
|
....
|
|
|
|
== Union
|
|
|
|
We can use the `//` operator to combine two attribute sets.
|
|
Attributes in the right-hand set take preference.
|
|
|
|
[source]
|
|
.Example
|
|
....
|
|
nix-repl> a = { x = 7; y = "hello"; }
|
|
|
|
nix-repl> b = { y = "wombat"; z = 3; }
|
|
|
|
nix-repl> a // b
|
|
{
|
|
x = 7;
|
|
y = "wombat";
|
|
z = 3;
|
|
}
|
|
....
|
|
|
|
This is often used to "modify" one or more values in the set.
|
|
Recall that Nix values are immutable, so the result is a new value (the original is not actually modified).
|
|
|
|
[source]
|
|
.Example
|
|
....
|
|
nix-repl> animal // { species = "tiger"; }
|
|
{ age = 10; name = { ... }; species = "tiger"; }
|
|
....
|
|
|
|
[#rec-attrset]
|
|
== Recursive attribute sets
|
|
|
|
An ordinary attribute set cannot refer to its own elements.
|
|
To do this, you need a _recursive_ attribute set.
|
|
|
|
[source]
|
|
.Example
|
|
....
|
|
nix-repl> { x = 3; y = 4*x; }
|
|
error: undefined variable 'x'
|
|
|
|
at «string»:1:16:
|
|
|
|
1| { x = 3; y = 4*x; }
|
|
| ^
|
|
|
|
nix-repl> rec { x = 3; y = 4*x; }
|
|
{ x = 3; y = 12; }
|
|
....
|
|
|
|
== Useful built-in functions for attribute sets
|
|
|
|
Nix provides some built-in functions for working with attribute sets;
|
|
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).
|
|
|
|
Get an alphabetical list of the keys.
|
|
|
|
[source]
|
|
.Example
|
|
....
|
|
nix-repl> builtins.attrNames animal
|
|
[ "age" "name" "species" ]
|
|
....
|
|
|
|
Get the values associated with each key, in alphabetical order by the key names.
|
|
|
|
[source]
|
|
.Example
|
|
....
|
|
nix-repl> builtins.attrValues animal
|
|
[ 10 "Professor Paws" "cat" ]
|
|
....
|
|
|
|
What value is associated with a key?
|
|
|
|
[source]
|
|
.Example
|
|
....
|
|
nix-repl> builtins.getAttr "age" animal
|
|
10
|
|
....
|
|
|
|
Does the set have a value for a key?
|
|
|
|
[source]
|
|
.Example
|
|
....
|
|
nix-repl> builtins.hasAttr "name" animal
|
|
true
|
|
|
|
nix-repl> builtins.hasAttr "car" animal
|
|
false
|
|
....
|
|
|
|
Remove one or more keys and associated values from a set.
|
|
|
|
[source]
|
|
.Example
|
|
....
|
|
nix-repl> builtins.removeAttrs animal [ "age" "species" ]
|
|
{ name = "Professor Paws"; }
|
|
....
|
|
|
|
Display an attribute set, including nested sets.
|
|
|
|
[source]
|
|
.Example
|
|
....
|
|
nix-repl> builtins.toJSON animal
|
|
"{\"age\":10,\"name\":{\"first\":\"Professor\",\"last\":\"Paws\"},\"species\":\"cat\"}"
|
|
....
|
|
|