lib.misc.mergeAttrByFunc

The following is the current implementation of this function.

mergeAttrByFunc =
    x: y:
    let
      mergeAttrBy2 =
        { mergeAttrBy = mergeAttrs; } // (maybeAttr "mergeAttrBy" { } x) // (maybeAttr "mergeAttrBy" { } y);
    in
    foldr mergeAttrs { } [
      x
      y
      (mapAttrs
        (
          a: v: # merge special names using given functions
          if x ? ${a} then
            if y ? ${a} then
              v x.${a} y.${a} # both have attr, use merge func
            else
              x.${a} # only x has attr
          else
            y.${a} # only y has attr)
        )
        (
          removeAttrs mergeAttrBy2
            # don't merge attrs which are neither in x nor y
            (filter (a: !x ? ${a} && !y ? ${a}) (attrNames mergeAttrBy2))
        )
      )
    ];