5.5 ONCE and AGAIN

The last part of a statement is an optional ONCE or AGAIN. ONCE specifies that the statement is self-abstaining or self-reinstating (this will be explained below); AGAIN specifies that the statement should behave like it has already self-reinstated or self-abstained. Whether the behaviour is self-abstention or self-reinstatement depends on whether the statement was initially abstained or not; a ONCE on an initially reinstated statement or AGAIN on an initially abstained statement indicates a self-abstention, and a ONCE on an initially abstained statement or AGAIN on an initially reinstated statement indicates a self-reinstatement.

The first time a self-abstaining statement is encountered, it is executed as normal, but the statement is then abstained from and therefore will not run in future. Likewise, the first time a self-reinstating statement is encountered, it is not executed (as is normal for an abstained statement), but then becomes reinstated and will run in future. In each of these cases, the ONCE effectively changes to an AGAIN; the ONCE only happens once, as might be expected.

REINSTATING a currently abstained self-abstaining statement or ABSTAINING (that is, with the ABSTAIN or REINSTATE commands) a currently reinstated self-reinstating statement causes the AGAIN on the statement to change back into a ONCE, so the statement will again self-abstain or self-reinstate. Likewise, REINSTATING a currently abstained self-reinstating statement or ABSTAINING a currently reinstated self-abstaining statement causes its ONCE to turn into an AGAIN.

Historical note: ONCE was devised by Malcom Ryan as a method of allowing synchronisation between threads in a multithreaded program (ONCE is atomic with the statement it modifies, that is, there is no chance that threads will change between the statement and the ONCE). AGAIN was added to Malcom Ryan’s Threaded Intercal standard on the suggestion of Kyle Dean, as a method of adding extra flexibility (and to allow the ONCEs to happen multiple times, which is needed to implement some multithreaded algorithms).