nixpkgs/pkgs/lib/attrsets.nix

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# Operations on attribute sets.
with {
inherit (builtins) head tail;
inherit (import ./default.nix) fold;
};
rec {
inherit (builtins) attrNames listToAttrs hasAttr isAttrs;
# Return an attribute from nested attribute sets. For instance ["x"
# "y"] applied to some set e returns e.x.y, if it exists. The
# default value is returned otherwise. !!! there is also
# builtins.getAttr (is there a better name for this function?)
getAttr = attrPath: default: e:
let attr = head attrPath;
in
if attrPath == [] then e
else if builtins ? hasAttr && hasAttr attr e
then getAttr (tail attrPath) default (builtins.getAttr attr e)
else default;
# ordered by name
attrValues = attrs: attrVals (__attrNames attrs) attrs;
attrVals = nameList : attrSet :
map (x: builtins.getAttr x attrSet) nameList;
# iterates over a list of attributes collecting the attribute attr if it exists
catAttrs = attr : l : fold ( s : l : if (hasAttr attr s) then [(builtins.getAttr attr s)] ++ l else l) [] l;
}