The indexing expression in MOO extracts a specified element from a list or string:
expression-1[expression-2]
First, expression-1 is evaluated; it must return a list or a string (the
sequence). Then, expression-2 is evaluated and must return an
integer (the index). If either of the expressions returns some other type
of value, E_TYPE is returned. The index must be between 1 and the
length of the sequence, inclusive; if it is not, then E_RANGE is raised.
The value of the indexing expression is the index'th element in the sequence.
Anywhere within expression-2, you can use the symbol $ as an
expression returning the length of the value of expression-1.
"fob"[2] => "o"
"fob"[1] => "f"
{#12, #23, #34}[$ - 1] => #23
Note that there are no legal indices for the empty string or list, since there are no integers between 1 and 0 (the length of the empty string or list).
Fine point: The
$expression actually returns the length of the value of the expression just before the nearest enclosing[...]indexing or subranging brackets. For example:"frob"[{3, 2, 4}[$]] => "b"