parallel clause

In Algol 68, there are several types of clauses ('block's in C terminology) into which statements can be combined:

• the serial clause: normal sequential execution
• the collateral clause: execution in some unpredictable order
• the parallel clause: execution at the same time

The parallel clause exists to allow truly parallel implementations. The effects on common data are completely unpredictable: no possible order of execution of statements may explain the outcome. For instance, the collateral clause (i +:= 1, i -:= 1) is guaranteed to have no net effect on the value of i; the corresponding parallel clause may change its value to anything, depending on the implementation.

The language offers semaphores as a synchronization mechanism.