Add small section about or expressions

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Vincent Ambo 2020-11-25 13:21:45 +01:00 committed by Vincent Ambo
parent 56a1f03a10
commit 3236b7ff28

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@ -23,7 +23,8 @@ important is missing.
- [`if ... then ... else ...`](#if--then--else-)
- [`inherit` keyword](#inherit-keyword)
- [`with` statements](#with-statements)
- [`import` / `NIX_PATH` / `<entry>`](#import--nixpath--entry)
- [`import` / `NIX_PATH` / `<entry>`](#import--nix_path--entry)
- [`or` expressions](#or-expressions)
- [Standard libraries](#standard-libraries)
- [`builtins`](#builtins)
- [`pkgs.lib`](#pkgslib)
@ -331,6 +332,31 @@ let pkgs = import <nixpkgs> {};
in pkgs.something
```
## `or` expressions
Nix has a keyword called `or` which can be used to access a value from an
attribute set while providing a fallback to a default value.
The syntax is simple:
```nix
# Access an existing attribute
let set = { a = 42; };
in set.a or 23
```
Since the attribute `a` exists, this will return `42`.
```
# ... or fall back to a default if there is no such key
let set = { };
in set.a or 23
```
Since the attribute `a` does not exist, this will fall back to returning the
default value `23`.
# Standard libraries
Yes, libraries, plural.