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111 lines
2.1 KiB
Text
111 lines
2.1 KiB
Text
= Attribute set operations
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== Selection
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The `.` operator selects an attribute from a set.
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[source]
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....
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nix-repl> animal = { name = { first = "Professor"; last = "Paws"; }; age = 10; species = "cat"; }
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nix-repl> animal . age
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10
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nix-repl> animal . name . first
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"Professor"
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....
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== Query
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We can use the `?` operator to find out if a set has a particular attribute.
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[source]
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....
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nix-repl> animal ? species
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true
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nix-repl> animal ? bicycle
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false
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....
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== Modification
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We can use the `//` operator to modify an attribute set.
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Recall that Nix values are immutable, so the result is a new value (the original is not modified).
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Attributes in the right-hand set take preference.
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[source]
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....
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nix-repl> animal // { species = "tiger"; }
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{ age = 10; name = { ... }; species = "tiger"; }
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....
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[#rec-attrset]
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== Recursive attribute sets
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An ordinary attribute set cannot refer to its own elements.
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To do this, you need a _recursive_ attribute set.
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[source]
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....
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nix-repl> { x = 3; y = 4*x; }
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error: undefined variable 'x'
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at «string»:1:16:
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1| { x = 3; y = 4*x; }
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| ^
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nix-repl> rec { x = 3; y = 4*x; }
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{ x = 3; y = 12; }
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....
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== Useful built-in functions for attribute sets
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Nix provides some built-in functions for working with attribute sets;
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a few examples are shown below.
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For more information on these and other built-in functions, see the Nix Manual
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(https://nixos.org/manual/nix/stable/language/builtins).
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Get an alphabetical list of the keys.
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[source]
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....
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nix-repl> builtins.attrNames animal
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[ "age" "name" "species" ]
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....
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Get the values associated with each key, in alphabetical order by the key names.
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[source]
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....
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nix-repl> builtins.attrValues animal
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[ 10 "Professor Paws" "cat" ]
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....
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What value is associated with a key?
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[source]
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....
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nix-repl> builtins.getAttr "age" animal
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10
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....
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Does the set have a value for a key?
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[source]
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....
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nix-repl> builtins.hasAttr "name" animal
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true
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nix-repl> builtins.hasAttr "car" animal
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false
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....
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Remove one or more keys and associated values from a set.
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[source]
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....
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nix-repl> builtins.removeAttrs animal [ "age" "species" ]
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{ name = "Professor Paws"; }
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....
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