Webel ‘def$’ DRY default capture! Applicable to arguments and options.

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[TECHNICAL SLIDE TRAIL] The Webel libraries for Wolfram Mathematica: With SysMLv1 models.
Section
SECTION: Webel coding and naming conventions for the Wolfram Language
Topic level
ESSENTIAL
Mathematica keywords
expression
Options
default
Webel Mathematica keywords
'def$arg$'
'def$opt$'
default helper
Keywords
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The value of a 'def$' default helper can be any Wolfram Language expression.

This convention and policy can be used standalone or in combination with the Webel '$opt$' (options) and '$arg$' (arguments) help holder conventions explored in detail in this trail section:

SECTION: The Webel '$opt$' and '$arg$' help holder conventions for options and arguments in the Wolfram Language
For reference, the associated policy notes (you don't need to visit these yet):
Webel: Mathematica: CONVENTION: Option help holders use a String variable with the prefix '$opt$', a related '$info$opt$' String, a 'def$opt$' default value expression, and optional '$warn$opt$', '$tip$opt$', '$lab$opt$', and 'type$opt$' indicator ...
Webel: Mathematica: CONVENTION: Argument help holders use a String variable with the prefix '$arg$', a related '$info$arg$' String, a 'def$arg$' default value expression and/or 'req$arg$', and optional '$warn$arg$', '$tip$arg$, '$lab$arg$', 'type$arg$'..
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Next trail section
SECTION: The Webel '$opt$' and '$arg$' help holder conventions for options and arguments in the Wolfram Language
Notes
[NAMING, OPTION, PATTERN, TIP]{OPTION} Mathematica: Webel: You CAN/MAY use $ in variable names and function names - just not as the first character before a Capital - and it's extremely useful. You won't get sent to Azkaban prison if do you use a $ character!
[NAMING, PATTERN, POLICY]{STRICT} Webel: Mathematica: CONVENTION: Option help holders use a String variable with the prefix '$opt$', a related '$info$opt$' String, a 'def$opt$' default value expression, and optional '$warn$opt$', '$tip$opt$', '$lab$opt$', and 'type$opt$' indicator ...
[NAMING, PATTERN, POLICY]{STRICT} Webel: Mathematica: CONVENTION: Argument help holders use a String variable with the prefix '$arg$', a related '$info$arg$' String, a 'def$arg$' default value expression and/or 'req$arg$', and optional '$warn$arg$', '$tip$arg$, '$lab$arg$', 'type$arg$'..
Snippets (quotes/extracts)
Related slides (includes other tutorials)
DRY not WET! "Don't Repeat Yourself!"
Webel ‘$arg$’, ‘def$arg$’, and ‘rule$arg$’ convention
Related slides (backlinks, includes other tutorials)
Visit also
The Webel SysML4Mathematica recipe for modelling Wolfram Language code in Systems Modeling Language (SysML) for Model-Based Systems Engineering
Visit also (backlinks)