From QB64.org wiki
$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
- Unlike LET, $LET is not optional.
- $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 valid, but $LET 3.2 = ""TRUE"" will error because of the double quotes.
- See example 1 in $IF.