L22) What is the exact semantics of leave P and restart P, when P is the name of a pattern?
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This article is from the FAQ, by with numerous contributions by others.
L22) What is the exact semantics of leave P and restart P, when P is the name of a pattern?
Leave and restart are the very basic local control mechanisms in BETA. Leave
and restart are specified by:
restart id
and
leave id
where id is the name of either a label or an enclosing pattern. When id is
an enclosing pattern, id is defined to refer to the do-part of id (hence not
to the do-part of any superpattern of id).
Consider the pattern:
P: A(# ...
do (* L1 *)
...;
leave/restart P;
...;
(* L2 *)
#)
restart P implies that execution continues at (* L1 *):
This means that restart P has the effect of entering the do-part
of P as after an inner in A.
leave P implies that execution continues at (* L2 *):
This means that leave P has the effect that execution continues in
the do-part of A after the inner that called the main-do-part of
P.
Example:
(# A: (#
do (for 4 repeat '['->put; INNER; ']'->put for)
#);
P: A (# k: @integer
do k+1->k->putInt;
(if k=2 then '-'->put; leave P if);
(if k=3 then '*'->put; restart P if);
'+'->put
#);
do P
#)
will give the following output:
[1+][2-][3*4+][5+]
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