leftHandSide // rightHandSide

In reply to @qyriad:katesiria.org
// isn't something like @ or ... which changes the semantics of something. // is just flat out an operator, like addition and subtraction. but rather than operating on integers (like + and -), // operates on attribute sets.

leftHandSide // rightHandSide

both side ("operands") must be attribute sets. the result of leftHandSide // rightHandSide is another attribute set, which has all the attributes in leftHandSide and all the attributes in rightHandSide, where overlapping values take precedence from rightHandSide

It's a dictionary merge, basically

In reply to @qyriad:katesiria.org
It's like the difference between lib.mkIf and lib.optionalAttrs, if you're familiar with those two

In reply to@grimmauld:grimmauld.de
wait what, whats the actual diff?

nix-repl> lib.mkIf true { foo = "bar"; }
{
  _type = "if";
  condition = true;
  content = { foo = "bar"; };
}
nix-repl> lib.mkIf false { foo = "bar"; }
{
  _type = "if";
  condition = false;
  content = { foo = "bar"; };
}

nix-repl> lib.optionalAttrs true { foo = "bar"; }
{ foo = "bar": }
nix-repl> lib.optionalAttrs false { foo = "bar"; }
{ }

mkMerge = contents:
{
  _type = "merge";
  inherit contents;
};

In reply to@grimmauld:grimmauld.de
it does work for all attrsets though, at least from my testing XD