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$LET is a 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 must follow QB64's variable naming conventions.
  • 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 if the user is running QB64 in a Windows environment.
    • LINUX if the user is running QB64 in a Linux environment.
    • MAC or MACOSX if the user is running QB64 in a macOS environment.
    • 32BIT if the user is running a 32-bit version of QB64.
    • 64BIT if the user is running a 64-bit version of QB64.
    • VERSION, which is set to the version of the QB64 compiler.


  • See example 1 in $IF.

See also

Keyword Reference - Alphabetical
Keyword Reference - By Usage
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