cognominal/conditional dereference #raku

## conditional dereference #raku
=head2 conditional dereference

A conditional dereference is a pattern matching and data structure
walking at once that blends well with laziness and functional
programming.

Conditional dereference are special dereference operators that work
as their not conditional counterpart if the dereference value des not
exist.  Otherwise, the C<<< postcircumfix:<< < >! >> >>>, operator
thows an exception  and the C<<< postcircumfix:<< < >? >> >>>
silently aborts.  In case of a silent abortion the flow proceeds
directly to the next statement, but if the conditional dereference
expression is evaluated in boolean context, its value is deemed to be
false and the statement proceeds according to that value.

Conditional dereference is useful in a function argument. An C<only>
subroutine or method is not called if any of its arguments evaluation
silently aborts. In such a case, it returns the undefined value of its
return type.
XXX What about multis?

If the value dereferenced with C<<< postcircumfix:<< =< > >> >>> does
not exist, the value is deemed to be the default value that would be
produced in that context.

Also, if the statement starts by a loop modifier and proceeds without
abortion, the control flow goes according to the said modifier.

The following example walks a parse tree to generate html
code. Whithin the loop, if the current node represents a header, that
is C<< .<head> >>  is defined, an html string is interpolated.  An
exception is thrown if C<< .<head><number> >> does not exist.
If C<< .head<content> >> is missing, the value produced by
C<< .=<content> >> is the default value expected in that context.

  for  (@array) {
     ...
     next $html .=  ".<h$nr>" ~ .=<content>? ~ "</$nr>"  if $nr = .<head>?<number>!
    ...
  }
	=head2 conditional dereference

	A conditional dereference is a pattern matching and data structure
	walking at once that blends well with laziness and functional
	programming.

	Conditional dereference are special dereference operators that work
	as their not conditional counterpart if the dereference value des not
	exist. Otherwise, the C<<< postcircumfix:<< < >! >> >>>, operator
	thows an exception and the C<<< postcircumfix:<< < >? >> >>>
	silently aborts. In case of a silent abortion the flow proceeds
	directly to the next statement, but if the conditional dereference
	expression is evaluated in boolean context, its value is deemed to be
	false and the statement proceeds according to that value.

	Conditional dereference is useful in a function argument. An C<only>
	subroutine or method is not called if any of its arguments evaluation
	silently aborts. In such a case, it returns the undefined value of its
	return type.
	XXX What about multis?

	If the value dereferenced with C<<< postcircumfix:<< =< > >> >>> does
	not exist, the value is deemed to be the default value that would be
	produced in that context.

	Also, if the statement starts by a loop modifier and proceeds without
	abortion, the control flow goes according to the said modifier.

	The following example walks a parse tree to generate html
	code. Whithin the loop, if the current node represents a header, that
	is C<< .<head> >> is defined, an html string is interpolated. An
	exception is thrown if C<< .<head><number> >> does not exist.
	If C<< .head<content> >> is missing, the value produced by
	C<< .=<content> >> is the default value expected in that context.

	for (@array) {
	...
	next $html .= ".<h$nr>" ~ .=<content>? ~ "</$nr>" if $nr = .<head>?<number>!
	...
	}