TEXT

try this:

CLEAR LOCAL
LOCAL FN testStringToHandleToString
  DIM myString$, myOtherString$
  DIM myStrSize%, myStrSize&
  DIM myStrHandle&, err%

  myString$    = "This be a test!"

  myStrSize%   = LEN(myString$)

  myStrSize&   = myStrSize%    'BLOCKMOVE and FN NEWHANDLE require a long&

  myStrHandle& = FN NEWHANDLE(myStrSize&)

  LONG IF myStrHandle&
    err% = FN HLOCK(myStrHandle&)

    BLOCKMOVE (@myString$ + 1), [myStrHandle&], myStrSize&

    myStrSize& = FN GETHANDLESIZE(myStrHandle&)
    myStrSize% = myStrSize&

    POKE @myOtherString$, myStrSize%
    BLOCKMOVE [myStrHandle&], (@myOtherString$ + 1), myStrSize&

    err% = FN HUNLOCK(myStrHandle&)

    DEF DISPOSEH(myStrHandle&)
  END IF

  PRINT myString$
  PRINT myOtherString$

END FN

'Will run as is
LOCAL FN str2Hndl(@theStr&,theHndl&)
  LONG IF theHndl&
    size = PEEK(theStr&)
    err = FN SETHANDLESIZE(theHndl&,size)
    BLOCKMOVE theStr&+1,[theHndl&],size
  END IF
END FN

LOCAL FN hndl2Str(theHndl&,@theStr&)
  LONG IF theHndl&
    size = FN GETHANDLESIZE(theHndl&)
    IF size > 255 THEN size = 255
    POKE theStr&, size
    BLOCKMOVE [theHndl&],theStr&+1,size
  END IF
END FN

DIM myStr$,myNewStr$

myStr$ = "  It works. Click to end."
myHndl& = FN NEWHANDLE(0)
LONG IF myHndl&
  FN str2Hndl(myStr$,myHndl&)
  FN hndl2Str(myHndl&,myNewStr$)
  PRINT myNewStr$
END IF

DO
UNTIL FN BUTTON

DEF DISPOSEH(myHndl&)

I modified your code a little to do more specifically what I wanted -- which was to create two functions that: 1) If given a string, would return a handle; 2) if given a handle, would return a string.

You might want to review the following code to see if I blundered.

Thanks again for the primer.

'--- will run as is --

COMPILE  0,_dimmedVarsOnly
END GLOBALS

'--------------------------

LOCAL FN buildWind
  WINDOW 1,"String <=> Handle",(0,0)-(300,100),_docNoGrow
END FN

'--------------------------

LOCAL FN str2Hndl(@theStr&)
  DIM size
  DIM err
  DIM theHndl&

  theHndl& = FN NEWHANDLE(0)
  LONG IF theHndl&
    size = PEEK(theStr&)
    err = FN SETHANDLESIZE(theHndl&,size)
    BLOCKMOVE theStr&+1,[theHndl&],size
  XELSE
    theHndl& = _nil
  END IF

END FN = theHndl&

'--------------------------

LOCAL FN hndl2Str$(theHndl&)
  DIM size
  DIM theStr$
  DIM theStr&

  LONG IF theHndl&
    theStr& = @theStr$
    size = FN GETHANDLESIZE(theHndl&)
    IF size > 255 THEN size = 255
    POKE theStr&, size
    BLOCKMOVE [theHndl&],theStr&+1,size
  XELSE
    theStr$ = ""
  END IF

END FN = theStr$

'--------------------------

"MAIN"

FN buildWind
DIM myStr$
DIM myNewStr$
DIM myHndl&

myStr$ = "  It works. Click to end."

myHndl&   = FN str2Hndl(myStr$)
myNewStr$ = FN hndl2Str$(myHndl&)

PRINT myNewStr$

DO
UNTIL FN BUTTON

DEF DISPOSEH(myHndl&)

Looks good to me. There are a couple of insignificant changes I would probably make, just because I'm passionate about tight code. (If you don't make the changes, I guarantee you'll never know the difference.)

1. If FN NEWHANDLE(0) fails (hard to imagine), theHndl& will already be _nil, so you don't really need
XELSE
theHndl& = _nil

2. As a matter of course I prefer to use
POKE @theStr$,0
instead of
theStr$ = ""
because it is faster, even though that often doesn't matter.

3. I would avoid a couple of lines and duplicate variable names by eliminating
theStr& = @theStr$
Just use @theStr$ where you have theStr&.

Here are the FNs with my edits. I've not tested them.

'--------------------------

LOCAL FN str2Hndl(@theStr&)
  DIM size
  DIM err
  DIM theHndl&

  theHndl& = FN NEWHANDLE(0)
  LONG IF theHndl&
    size = PEEK(theStr&)
    err = FN SETHANDLESIZE(theHndl&,size)
    BLOCKMOVE theStr&+1,[theHndl&],size
  END IF

END FN = theHndl&

'--------------------------

LOCAL FN hndl2Str$(theHndl&)
  DIM size
  DIM theStr$

  LONG IF theHndl&
    size = FN GETHANDLESIZE(theHndl&)
    IF size > 255 THEN size = 255
    POKE @theStr$, size
    BLOCKMOVE [theHndl&],@theStr$+1,size
  XELSE
    POKE @theStr$, 0
  END IF

END FN = theStr$

'--------------------------