$LET

$LET is a precompiler metacommand. It is used to define or redefine precompiler variables for use in the $IF...$ELSE...$END IF block statements.

Syntax

$LET variable = value

Description

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 must follow QB64's variable naming conventions. They will be capitalized automatically.
Values may contain any number of periods to separate numbers or words in a string, e.g. in version numbers such as 3.14.1 or strings like MARY.HAD.A.LITTLE.LAMB etc..
- Note that strings may not contain spaces and therefore may be given without leading/trailing quotes.
You can check a precompiler variable against special values DEFINED and UNDEFINED, in order to assess whether the variable has already been assigned a value. Useful for code in libraries which may be repeated.
The precompiler comes with some preset values which can be used to help determine which code blocks to include/exclude. These are:
- WIN or WINDOWS is true(-1) if the user is running QB64 in a Windows environment, it is false(0) otherwise.
- LINUX is true(-1) if the user is running QB64 in a Linux environment, it is false(0) otherwise.
- MAC or MACOSX is true(-1) if the user is running QB64 in a macOS environment, it is false(0) otherwise.
- 32BIT is true(-1) if the user is running a 32-bit version of QB64., it is false(0) otherwise.
- 64BIT is true(-1) if the user is running a 64-bit version of QB64., it is false(0) otherwise.
- VERSION, which is set to the version of the QB64 compiler.
Some new presets have been introduced with QB64-PE v4.0.0, these are:
- _QB64PE_ is always true(-1), it indicates the use of the QB64 Phoenix Edition compiler at least v.4.0.0
- _ASSERTS_ is one(1) if $ASSERTS or $ASSERTS:CONSOLE is used, it is zero(0) otherwise.
- _CONSOLE_ is one(1) if a console is active either by using $CONSOLE directly or implied by $ASSERTS:CONSOLE, it is two(2) if $CONSOLE:ONLY is set, it is zero(0) if no console is available (both console variants may appear multiple times in a program, the last found one determines the final state).
- _DEBUG_ is one(1) if $DEBUG is used, it is zero(0) otherwise.
- _EXPLICIT_ is one(1) if the program uses OPTION _EXPLICIT, it is zero(0) otherwise (note OE also implies OPTION _EXPLICITARRAY).
- _EXPLICITARRAY_ is one(1) if the program uses OPTION _EXPLICITARRAY or OPTION _EXPLICIT, it is zero(0) otherwise.

Important notes regarding the preset values

Although there's for simplicity no internal protection against it, don't overwrite the presets using $LET, always consider the presets as read-only.
The presets shall mainly serve the ability, for library makers, to check what features are active in a program, don't misuse it to enforce certain features, e.g.

don't check for _CONSOLE_ and force to open one, if none is there already
don't check for _EXPLICIT_ and force it, if it's not in effect already

Think of it like checking for WINDOWS and if it's not forcing the user to buy a Windows system or checking for 32BIT and if it's not forcing the user to downgrade his 64-bit system. You wouldn't do that, right? And by that you should not enforce things, which the user did not use in his program already.