How to get value return from a function within a loop?

I am trying to work out how to test the return value of a function call within a loop and breaks the loop if function returns a specific value.

The follow piece of batch script is my attempt, the function return expected value on the first 2 function calls. When the call is made within a loop, I cannot figure out how to test the return value. Please point me to a direction on how to test the function return value within the loop.

SET var1=2
SET var2=0
CALL :FUNC %var1 var2
ECHO var2 is: %var2%

REM ============================

SET var1=6
SET var2=0
CALL :FUNC %var1 var2
ECHO var2 is: %var2%

REM ============================

SETLOCAL EnableDelayedExpansion
SET list=1 0 3 4
FOR %%n IN (%list%) DO (
    CALL :FUNC %%n rtn
    IF !rtn! == 0 (
        GOTO DONE
    )
)
ECHO rtn is: %rtn%

: DONE
PAUSE
GOTO :eof


REM %1 is an in parameter
REM %2 is an out parameter
: FUNC
SETLOCAL EnableDelayedExpansion
SET var=%1
EndLocal & SET %2=%var% & GOTO :eof