$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.

Availability

• 

  v1.0

• 

  all

• 
• 

  yes

• 

  yes

• 

  yes

• In QB64-PE v4.0.0 several new presets got added into the precompiler (see above).

Examples

• See example 1 in $IF.

See also

• $IF
• $ELSE
• $ELSEIF
• $END IF

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