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Control Statements

A project log for Ternary Computing Menagerie

A place for documenting the many algorithms, data types, logic diagrams, etc. that would be necessary for the design of a ternary processor.

Mechanical AdvantageMechanical Advantage 04/29/2019 at 07:190 Comments

Control statements such as While loops, Do-While loops, For loops, Switch statements and If statements are most of the reason you have relational or logical operations in the first place. Looking at how balanced ternary values fit into this system of program flow is of the highest importance.

To put it simply, the existing control flow statements should work the same as always. Nothing would stop you from using kleene data types or the spaceship operator for the decision making, but it doesn't actually change the way the statements work.

If variables a and b are kleenes, then:

while ((a <=> b) >= NEUT)...

is equivalent to:

while ((a >= b))...

No new ground is being broken here, they would just work as you would normally expect them to. Of course existing control statements are expecting boolean evaluation so they would need to do type checking to correctly handle kleene FALSE because it would look like a boolean TRUE due to being non-zero.

All this doesn't mean that control statements that take advantage of the more expressive ternary logic couldn't be created. An alternative to the "if" statement is easy to work out.

Imagine a conveyor belt that could be moving forward, moving backward, or stopped. The variable that held the current conveyor motion is a kleene called "conveyorDirection" that contains a - (FALSE) for reverse, a 0 (NEUT) for stopped, or a + (TRUE) for forward. Now imagine a control statement that natively evaluated kleenes. We'll call it an evaluation statement or "eval" statement.

eval(conveyorDirection)
{
     //Code if -
}
{
     //Code if 0
}
{
     //Code if +
}

The syntax could be different as long as it was clear which section of code would be executed for each state.

This could even be used to ease debugging and error handling. Imagine you were evaluating a game variable called lifePoints. This variable should never be negative. In a boolean system, if an off-by-one bug or manipulation resulted in underflow, the negative non-zero value would return TRUE, so additional steps would have to be taken to check for this. With this hypothetical eval statement you have a convenient place to put your error handler call or even a breakpoint for your debugger.

eval(lifePoints <=> 0)
{
     //Code if -
     invalidValueHandler();
     //Feel free to put breakpoint here as well
}
{
     //Code if 0
     printf("You Died!");
}
{
     //Code if +
     printf("%d remaining", lifePoints);
}

An alternative while loop is a bit more complex. Currently, while loops test an expression and if it returns TRUE, executes the code, then tests the expression again, and so on. Any combination of TRUE, NEUT, and FALSE could be manipulated with conditional operators to evaluate TRUE, so the while loop (and do-while loop by extension) can already handle any three-value expression you want to throw at it.

A natively ternary version of the while loop might be something that evaluates an expression and then chooses which code to loop through. Let's call this hypothetical control statement a "choice loop". The distinguishing feature is that it will switch back and forth through different loops (scary, right?) depending on the return value of the expression. If it returns FALSE, one section of code is looped through until it returns something else at which point it loops through that code, and so on. A break in one of the sections would release the loop. Here's an example.

choice(x <=> 0)
{
     //Loop if +
     doSomethingRepeatedly...
     x--                         //This loop executes repeatedly and decrements x
}
{
     //Loop if 0
     doSomething...
     break                  //This loop executes once when x reaches 0, then breaks
}
{
     //Loop if -
     doSomethingElse...
     break                 //This loop does something else if x returns FALSE, then breaks
}

If x was a positive number the above code would execute the TRUE loop over and over, decrementing x until it reached 0. Then it would execute the NEUTRAL loop once and break. If x was already 0, it would execute the NEUTRAL loop once and break. If x was negative, it would do some other action and break. Or you could have it do nothing and break as a simple fail-through.

Infinite loops would naturally have to be watched out for just like with any other loop statement but I think debugging such errors would end up being easier. Just put a break point at the beginning of each of the three loops and watch the variable being evaluated.

This control statement could be used to do pretty much anything you would expect from a for loop or a while loop. Combining it with a "do" statement would get a bit complex but could be arranged. It wouldn't replace a switch statement since that can be used to define an arbitrary number of cases.

On the surface the eval statement and choice loop look like elegant, native-ternary flow control statements. At least they look like that to me. Someone who actually has some language design experience would be better suited to judge them or to recommend alternatives.

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