My very naive implementation of IP6 address expansion in nix language

ip6.nix

/*

$ nix-instantiate --eval --expr 'with (import <nixpkgs> {}); with lib; builtins.toJSON ((import ./ip6.nix { lib = pkgs.lib; }).expandIP6 "ea5f::")' | jq -r . | jq .
"ea5f:0000:0000:0000:0000:0000:0000:0000"
$ nix-instantiate --eval --expr 'with (import <nixpkgs> {}); with lib; builtins.toJSON ((import ./ip6.nix { lib = pkgs.lib; }).expandIP6 "::ea5f")' | jq -r . | jq .
"0000:0000:0000:0000:0000:0000:0000:ea5f"
$ nix-instantiate --eval --expr 'with (import <nixpkgs> {}); with lib; builtins.toJSON ((import ./ip6.nix { lib = pkgs.lib; }).expandIP6 "ea5f::ff")' | jq -r . | jq .
"ea5f:0000:0000:0000:0000:0000:0000:00ff"

*/

{ lib }:
with lib;
with builtins;
rec {
  inherit (import ./lists.nix   { inherit lib; }) padListLeft padListRight repeatListItem;
  inherit (import ./strings.nix { inherit lib; }) padStringLeft;

  expandIP6 = v:
    let
      octetsSep = ":";
      octets = splitString octetsSep v;
      octetsNum = 8;
      octetLength = 4;
      paddedOctets =
        if head octets == "" then padListLeft  octetsNum "" octets else
        if last octets == "" then padListRight octetsNum "" octets else
        foldl (a: v: # will break if find two "", so address requires additional validation
          if v == ""
          then (a ++ [v] ++ (repeatListItem (octetsNum - (length octets)) v))
          else (a ++ [v])
        ) [] octets;
    in concatStringsSep
      octetsSep
      (map (padStringLeft octetLength "0") paddedOctets);
}

lists.nix

{ lib }:
with lib; rec {
  padListLeft = padSize: padWith: v:
    if length v < padSize
    then (padListLeft padSize padWith ([ padWith ] ++ v))
    else v;

  padListRight = padSize: padWith: v:
    if length v < padSize
    then (padListRight padSize padWith (v ++ [ padWith ]))
    else v;

  repeatListItem = times: v:
    (
      if times > 1
      then (repeatListItem (times - 1) v)
      else []
    ) ++ [v];
}

strings.nix

{ lib }:
with lib; rec {
  padStringLeft = padSize: padWith: v:
    if stringLength v < padSize
    then (padStringLeft padSize padWith "${padWith}${v}")
    else v;

  padStringRight = padSize: padWith: v:
    if stringLength v < padSize
    then (padStringRight padSize padWith "${v}${padWith}")
    else v;
}