On PEP 312: simple implicit lambda

[Stuart D. Gathman]

On Sun, 16 Feb 2003 09:47:11 -0500, Christos TZOTZIOY Georgiou wrote:


> What I would do in such a case, would be the following:
> 
> <read_more_data>:
>     data = file_object.read(4096)
> while data:
> 	# process here
>     <read_more_data>
> 
> The syntax I imagined is the one proposed above, with angle brackets and
> all.  What happens is this:

I did a much cleaner syntax of this idea for an assembler preprocessor.
Here is what it would look like in Python:

  while when:
    data = file_object.read(4096)
  then data:
    # process here

The idea is that WHEN introduces arbitrary code which "sets up" for
testing a condition, and THEN gets back to testing the condition.  When
can also be used with 'if', and is useful when combined with short circuit
evaluation.  Here is an example of the assembler code (IBM Series/1).  The
WHEN cause lets me issue multiple statements to setup for the CFNEN (like
Intel "string" insns).  The DO R6 provides an upper bound on the number of
loop iterations (an error checking feature nearly as useful as bounds
checking).

             DO R6,WHILE,WHEN,REG=R6
               MVWS  BTCBLBUF,R5       COULD PSD AND USE R1 FOR THIS
	       MVW   R4,R2
               MVW   R3,R7
               CFNEN (R5),(R2)
             THEN LLT
               AW DLEN,R4              POINT R4 TO NEXT RECORD
	     ENDDO

One final asm example to show how WHEN is used with short circuiting
AND/OR:

         IF (BTCBFLAG,OFF,BTCB at MWA+BTCB@MOV,TWI)
           CALL BTBUF,(BTCBNODE,0)
         ELSEIF (BTCBFLAG,ON,BTCB at MOV,TWI),OR,WHEN
           CALL DISPOSE,TYPE=X
           MVA  BTCBNODE,R4                      TRY TO AVOID RETRACE;
           CALL GETBLK,TYPE=X                    GET BLOCK LAST SEEN IN
           MVWS R3,BTCBABUF                      AND CHECK: 
	   MVW  R3,#NODE
         THEN (BTROOT,NE,BTCBROOT,CD),OR                   SAME FILE
	 THEN (R4,P,BTSON,MVD),OR,WHEN                     LEAF NODE
           IF (R4,Z,BTCOUNT,MVWS),THEN,POSTEOF             NON EMPTY
         THEN (R7,NP,BTCBSLOT,MVWS),OR,(R7,GT,R4),OR,WHEN  VALID POS
           CALL CHKKEY,TYPE=J                              MATCHING KEY
         THEN NE,THEN
           CALL BTTRACE,(BTCBKLEN)              ELSE, RETRACE
         ENDIF

The (op1,cond,op2,opcode) construct was short for
	WHEN opcode op2,op1
	THEN cond

In python syntax, that would be:

	 if (btcb.flag & (BTCB.MWA | BTCB.MOV)) != 0:
	   btbuf(btcb.node,None)
	 elif (btcb.flag & BTCB.MOV) != 0 or when:
	   dispose()
	   btcb.abuf = node = getblk(btcb.node)
	 then btroot != btcb.root or btson > 0 or when:
	   r4 = btcount
	   if r4 == 0: raise EOFException
	   r7 = btcb.slot
 	 then r7 <= 0 or r7 > r4 or when:
	   rc = chkkey()  # OK, this is artificial to mirror the ASM code
	 then rc:
	   bttrace(btcb.klen)

Flame away!  I've always liked it because it allows any sequence of statements, 
including nested loops and conditionals, to setup conditions in loops and
short-circuit boolean expressions.

-- 
	      Stuart D. Gathman <stuart at bmsi.com>
Business Management Systems Inc.  Phone: 703 591-0911 Fax: 703 591-6154
"Confutatis maledictis, flamis acribus addictis" - background song for
a Microsoft sponsored "Where do you want to go from here?" commercial.