$LET: Difference between revisions

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(Created page with "$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. {{PageSyntax}} : $LET variable = expression {{PageDescription}} * 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...")
 
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[[$LET]] is precompiler command, which is now usable by modern day [[cavemen]] to help include and exclude which sections of code compiles in their program based on OS/bit-size or other predefined conditions.
[[$LET]] is a precompiler [[Metacommand|metacommand]]. It is used to define or redefine precompiler variables for use in the [[$IF]]...[[$ELSE]]...[[$END IF]] block statements.




{{PageSyntax}}
{{PageSyntax}}
: [[$LET]] variable = expression
: [[$LET]] variable = value




{{PageDescription}}
{{PageDescription}}
* Unlike [[LET]], [[$LET]] is not optional.
* Unlike [[LET]], [[$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.
* $LET a = 12 sets a precompiler variable "a" to the value of 12. This variable is only valid for the precompiler itself and does nothing to affect the values of any variable/constant which might also be called "a" in the program.
* Variable names can contain numbers, letters, and periods in any order. [[$LET]] '''3.2 = TRUE''' is a perfectly valid variable and expression.
* Variable names must follow QB64's variable naming conventions. They will be capitalized automatically.
* 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.
* 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|$ASSERTS:CONSOLE]] is used, it is ''zero(0)'' otherwise.
** '''_CONSOLE_''' is ''one(1)'' if a console is active either by using [[$CONSOLE]] directly or implied by [[$ASSERTS|$ASSERTS:CONSOLE]], it is ''two(2)'' if [[$CONSOLE|$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 [[$DEBUG]] is used, it is ''zero(0)'' otherwise.
** '''_EXPLICIT_''' is ''one(1)'' if the program uses [[OPTION EXPLICIT|OPTION _EXPLICIT]], it is ''zero(0)'' otherwise (note OE also implies '''OPTION _EXPLICITARRAY''').
** '''_EXPLICITARRAY_''' is ''one(1)'' if the program uses [[OPTION EXPLICITARRAY|OPTION _EXPLICITARRAY]] or [[OPTION EXPLICIT|OPTION _EXPLICIT]], it is ''zero(0)'' otherwise.
; Important notes regarding the preset values
:* All presets are {{Text|'''read-only'''|red}} values and cannot be redefined by using [[$LET]] on it.
:* The presets shall just serve the ability, for library makers, to easily check what features are active in a program, they shall otherwise not be {{Text|'''misused to enforce'''|red}} certain features, e.g.
::* don't check for '''_CONSOLE_''' and force one to open, if none is there,
::* don't check for '''_EXPLICIT_''' and force it, if it's not in effect, etc.
:* 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 means you also should not enforce things, which the user did not already use in his program.
 
 
{{PageAvailability}}
<!-- QB64 = a version or none, QBPE = a version or all, Platforms = yes or no -->
<gallery widths="48px" heights="48px" mode="nolines">
File:Qb64.png|'''v1.0'''
File:Qbpe.png|'''all'''
File:Apix.png
File:Win.png|'''yes'''
File:Lnx.png|'''yes'''
File:Osx.png|'''yes'''
</gallery>
<!-- Additional availability notes go below the gallery, e.g. -->
* In '''QB64-PE v4.0.0''' several new presets got added into the precompiler (see above).




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* [[$ELSEIF]]
* [[$ELSEIF]]
* [[$END IF]]
* [[$END IF]]
* [[Cavemen]]




{{PageNavigation}}
{{PageNavigation}}

Latest revision as of 11:02, 18 November 2024

$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
  • All presets are read-only values and cannot be redefined by using $LET on it.
  • The presets shall just serve the ability, for library makers, to easily check what features are active in a program, they shall otherwise not be misused to enforce certain features, e.g.
  • don't check for _CONSOLE_ and force one to open, if none is there,
  • don't check for _EXPLICIT_ and force it, if it's not in effect, etc.
  • 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 means you also should not enforce things, which the user did not already use in his program.


Availability

  • v1.0

    v1.0

  • all

    all

  • Apix.png
  • yes

    yes

  • yes

    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


Navigation:
Main Page with Articles and Tutorials
Keyword Reference - Alphabetical
Keyword Reference - By usage
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