Mathematica: Webel: ADT pseudo classes: POLICY: The expression '$$' is reserved as the primary named parameter for the defining pattern of all ADT classes that extend the universal base 'ADT' class.

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Note kind
POLICY
Policy level
STRICT
Keywords
Example: 
ADT$Map[$$_Association]
ADT$Map /: get[$$_Association] := $$;

In the Webel recipe for ADTs this is actually inherited from a base ADT and uses a template 'pattern' variable that is passed to a definer function: 

ADT /: get[pattern] := $$;

So in the above case pattern would be $$_Associaton.

This policy offers a clearly identified variable expression that is not likely to otherwise be used in code (unless deliberately) and helps avoid this possible clash:

Mathematica: GOTCHA: The Module/With system (especially under rewriting when nested) can leave variables with a trailing '$' suffix, which may cause name clashes because the variables (Expressions) are available later outside the scope of the Module/With!

However, to make the Webel ADT recipe work when referenced from OTHER packages, you need this:

Mathematica: Webel ADT pseudo classes: A tricky POLICY & CONVENTION: Client packages MUST access the special "wrapped" primary named ADT-parameter '$$' via $ContextAliases! The recommended alias is "A`" (which stands of course for ADT).
Relates to
[STUB] TRAIL: Mathematica: The Webel recipe for Abstract Data Type (ADT) pseudo classes with inheritance for the Wolfram Language
Related notes
[GOTCHA, TIP]{INFORMATIVE} Mathematica: GOTCHA: The Module/With system (especially under rewriting when nested) can leave variables with a trailing '$' suffix, which may cause name clashes because the variables (Expressions) are available later outside the scope of the Module/With!
[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!
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