Conditional Dataflow

More general functions require conditional constructs. C and Java have if and switch statements, and ?: expressions. Lisp has cond, if, when, unless, and case expressions. In C, Java, and Lisp, these alter the flow of control. Full Metal Jacket doesn't have flow of control, however. It has flow of data.

Conditional dataflow in Full Metal Jacket is done using when+ and unless+ vertices. Each takes a boolean value, and a number of other values, at its inputs. when+ outputs the other values if the first is true (any value other than NIL), and outputs no values if it is false (NIL). unless+ behaves in the opposite manner, outputting if its first value is NIL.

myAbs is a function which returns the absolute value of a Real number:


Two separate calls were made, in the order shown in the transcript.

In the first call, the < vertex returned NIL, so the when+ vertex did not transmit any value to the - vertex, which never executed. The unless+ vertex transmitted myAbs's input to the outgate.

In the second call, the < vertex returned T, so the when+ vertex transmitted myAbs's input to the - vertex, which negated it before transmitting it to the outgate. The unless+ vertex did not transmit any value to the outgate.

abs call abs output


Conditional vertices, as well as those used for iteration, are yellow. Incidentally, when - is given a single input, as is done here, it negates it; otherwise, it subtracts all other inputs from its first input.

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