Windows .bat/.cmd function library in own file?

There is an easier way to load the library functions each time the main file is executed. For example:

@echo off
rem If current code was restarted, skip library loading part
if "%_%" == "_" goto restart
rem Copy current code and include any desired library
copy /Y %0.bat+lib1.bat+libN.bat %0.full.bat
rem Set the restart flag
set _=_
rem Restart current code
%0.full %*
:restart
rem Delete the restart flag
set _=
rem Place here the rest of the batch file
rem . . . . .
rem Always end with goto :eof, because the library functions will be loaded
rem after this code!
goto :eof

It's possible, and there are some different ways to do it.

1) Copy&Paste the complete "Library" into each of your files Works, but it's not really a library, and it's a horror to change/correct a library function in all files

2) include a library via call-wrapper

call batchLib.bat :length result "abcdef"

and batchLib.bat starts with

call %* 
exit /b
...
:length
...

Easy to program, but very slow, as each library call loads the library batch, and possible problems with the parameters.

3) A "self-loading" library BatchLibrary or how to include batch files _(cached)

It creates each time a temporary batch file, combined of the own code and the library code.
It do some advanced functions at the library startup like secure parameter access. But in my opinion it's also easy to use

A user script sample

@echo off
REM 1. Prepare the BatchLibrary for the start command
call BatchLib.bat

REM 2. Start of the Batchlib, acquisition of the command line parameters, activates the code with the base-library
<:%BL.Start%

rem  Importing more libraries ...
call :bl.import "bl_DateTime.bat"
call :bl.import "bl_String.bat"

rem Use library functions
call :bl.String.Length result abcdefghij
echo len=%result%

EDIT: Another way is ...

4) A macro library

You could use batch-macros, it's easy to include and to use them.

call MacroLib.bat

set myString=abcdef
%$strLen% result,myString
echo The length of myString is %result%

But it's tricky to build the macros!
More about the macro technic at Batch "macros" with arguments _(cached)

MacroLibrary.bat

set LF=^


::Above 2 blank lines are required - do not remove
set ^"\n=^^^%LF%%LF%^%LF%%LF%^^"
:::: StrLen pString pResult
set $strLen=for /L %%n in (1 1 2) do if %%n==2 (%\n%
        for /F "tokens=1,2 delims=, " %%1 in ("!argv!") do (%\n%
            set "str=A!%%~2!"%\n%
              set "len=0"%\n%
              for /l %%A in (12,-1,0) do (%\n%
                set /a "len|=1<<%%A"%\n%
                for %%B in (!len!) do if "!str:~%%B,1!"=="" set /a "len&=~1<<%%A"%\n%
              )%\n%
              for %%v in (!len!) do endlocal^&if "%%~b" neq "" (set "%%~1=%%v") else echo %%v%\n%
        ) %\n%
) ELSE setlocal enableDelayedExpansion ^& set argv=,