lib.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))
            )
      )
    ];