Translator – Data Types and Operators

Three data types are currently planned: double precision (the default), integers and strings. Several operators accept all three as operands (plus, equality, relational and assignment). Consider the different possible operand combinations for the plus operator (the default data type is double, % is for integers and $ is for strings):

A + B
A + J%
I% + B
I% + J%
S$ + T$

Any combination between a number and a string is invalid and an error needs to be reported. However, doubles and integers may be mixed in an expression. The expression will remain integer for efficiency as long as there are integer operands. In a double expression, integers will be promoted (converted) to double.

If the plus operator was implemented as a single routine, at run-time it would have to look at the operands and then decide what action to take, whether to add two doubles, whether to convert one operand or the other from integer to double and then add two doubles, whether to add two integers, or whether to concatenate two strings. All this decision making will slow execution.

These decisions will of course be made before run-time by the Translator. The Translator will know which operation will be needed at run-time, so it will add the appropriate code to the RPN list. Since there are five different possible combinations, that would mean five different add operator codes. If this same thing was implemented for every operator, then the number of codes and the number of execution routines would be excessive.

There are really only three operations, adding doubles, adding integers and concatenating strings. For the other two combinations, one or the other operand needs to be converted from integer to double. To accomplish this, there will be special hidden conversion codes (which have already been mentioned in several earlier posts). With these hidden codes, the above expressions would be converted to RPN as:

A B Add
A J% CvtDbl Add
I% CvtDbl B Add
I% J% AddInt
S$ T$ CatStr

The Add handles doubles, AddInt handles integers, and CatStr (concatenate) handles strings. The CvtDbl code converts it's integer operand to double. For now, the focus will be on doubles and integers; strings require more involved handling and will be added later. Next, data types and internal functions...