15.6.2.1015 CS-PICK c-s-pick TOOLS EXT

Interpretation:

Interpretation semantics for this word are undefined.

Execution:

( C: dest_u ... orig₀ | dest₀ -- dest_u ... orig₀ | dest₀ dest_u ) ( S: u -- )

Remove u. Copy dest_u to the top of the control-flow stack. An ambiguous condition exists if there are less than u+1 items, each of which shall be an orig or dest, on the control-flow stack before CS-PICK is executed.

If the control-flow stack is implemented using the data stack, u shall be the topmost item on the data stack.

See:

A.15.6.2.1015 CS-PICK.

Rationale:

The intent is to copy a dest on the control-flow stack so that it can be resolved more than once. For example:

\ Conditionally transfer control to beginning of
\ loop. This is similar in spirit to C's "continue"
\ statement.

: ?REPEAT ( dest -- dest ) \ Compilation
( flag -- ) \ Execution
0 CS-PICK POSTPONE UNTIL
; IMMEDIATE

: XX ( -- ) \ Example use of ?REPEAT
   BEGIN
     ...
   flag ?REPEAT ( Go back to BEGIN if flag is false )
     ...
   flag ?REPEAT ( Go back to BEGIN if flag is false )
     ...
   flag UNTIL ( Go back to BEGIN if flag is false )
;

Testing:

: ?repeat
0 CS-PICK POSTPONE UNTIL
; IMMEDIATE

VARIABLE pt4

: <= > 0= ;

T{ : pt5  ( n1 -- )
      pt4 !
      BEGIN
        -1 pt4 +!
        pt4 @ 4 <= ?repeat    \ Back to BEGIN if false
        111
        pt4 @ 3 <= ?repeat
        222
        pt4 @ 2 <= ?repeat
        333
        pt4 @ 1 =
      UNTIL
    ; -> }T

T{ 6 pt5 -> 111 111 222 111 222 333 111 222 333 }T

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UlrichHoffmann [66] CS-DROP (revised 2018-08-20) Proposal2018-08-20 20:22:25

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UlrichHoffmann [113] CS-DROP (revised 2019-08-22)Proposal2019-09-06 08:24:28

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AntonErtl 2019-09-06 08:54:22

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