nathan/nixpkgs
1
0
Fork 0
mirror of https://github.com/NixOS/nixpkgs.git synced 2026-06-05 21:03:40 +00:00
Code Issues Packages Projects Releases Wiki Activity

lib.attrsets.filterAttrs: Add tips / guidance (#428330)

Browse Source
This commit is contained in:
Robert Hensing
2025-08-10 13:25:49 +02:00
committed by GitHub
parent f3d769a3bb 77c2981a7d
commit f5d1677b8e
1 changed files with 10 additions and 0 deletions
Download Patch File Download Diff File Expand all files Collapse all files

10
lib/attrsets.nix
View File

@@ -633,6 +633,16 @@ rec {
: Predicate taking an attribute name and an attribute value, which returns `true` to include the attribute, or `false` to exclude the attribute.
<!-- TIP -->
If possible, decide on `name` first and on `value` only if necessary.
This avoids evaluating the value if the name is already enough, making it possible, potentially, to have the argument reference the return value.
(Depending on context, that could still be considered a self reference by users; a common pattern in Nix.)
<!-- TIP -->
`filterAttrs` is occasionally the cause of infinite recursion in configuration systems that allow self-references.
To support the widest range of user-provided logic, perform the `filterAttrs` call as late as possible.
Typically that's right before using it in a derivation, as opposed to an implicit conversion whose result is accessible to the user's expressions.
`set`
: The attribute set to filter