$LET is precompiler command, which is now usable by modern day cavemen to help include and exclude which sections of code compiles in their program based on OS/bit-size or other predefined conditions.
- $LET variable = expression
- $LET a = 12 sets a precompiler variable "a" to the value of 12. This variable is ONLY valid for the precompiler itself and does NOTHING to affect the values of any variable/constant which might also be called "a" in the program.
- Variable names can contain numbers, letters, and periods -- in any order! $LET 3.2 = TRUE is a perfectly valid variable and expression.
- Expressions can contain one set of leading and/or trailing quotes; and any number of numbers, letters, and periods, in any order. $LET 3.2 = "TRUE" is also perfectly, but $LET 3.2 = ""TRUE"" will error. (See the double quotes?)
Explaination: If you look at the code above, you'll see that we have the same CONST defined inside the program. Normally, we get an error if we try to define a CONST more than once, but the $IF condition here is CHOOSING which CONST we want inside our program.
AS long as Screenmode is 0, the program will exclude the code where CONST Red is defined as color 4. If Screenmode is 32, CONST Red will be defined as _RGB32(255, 0, 0).
The $LET and $IF statements let us control the code that actually gets compiled, while excluding the other blocks completely.