Nix Language Quirks: Difference between revisions
Merge the conditional key names entry just I added with the existing using null as a key name, and just add the more clear example of using conditionals |
Change the title of the ${null} attribute set entry since delete is misleading. Also link to the official documentation about it |
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== | == Attribute set entries with a name that evaluates to null will not be added to the set == | ||
From [https://nix.dev/manual/nix/2.24/language/syntax#attrs-literal this] section of the Nix Reference Manual: | |||
nix- | |||
{ | <blockquote> | ||
</ | In the special case where an attribute name inside of a set declaration evaluates to null (which is normally an error, as null cannot be coerced to a string), that attribute is simply not added to the set: | ||
{ ${if foo then "bar" else null} = true; } | |||
This will evaluate to {} if foo evaluates to false. | |||
</blockquote> | |||
The relevant source can be found [https://github.com/NixOS/nix/blob/26c3fc11eada3fa7df0284190095868a947fefe2/src/libexpr/eval.cc#L1249-L1250 here]. | |||
This can be | This feature can be used to conditionally include or exclude attribute set entries, for example: | ||
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This might used as an alternative to conditionally merging attribute sets using <code>//</code> like the following: | |||
<syntaxHighlight lang=nix> | |||
{ a = "b"; } // (if true then { foo = "bar"; } else { } ) | |||
</syntaxHighlight> | |||
== Nix Language FAQ == | == Nix Language FAQ == | ||