Mnemonics

"array", "box", "square"

• main use is for arrays
• also used in meta operators
• it generates "square" things (as in fuddy, old-fashioned, plain, vanilla)

Used as a brace pair or quote pair

• [] in a term treats it as an array
• [] instead of () around list values creates an Array object and can explicitly make a list from a single value
• [] around a infix operator, in list term position, creates a reduce meta operator
• [ ] around infix operators, in infix position, can be used to disambiguate and nest metaoperators
• & [ ] may be used as a shorthand for the extended identifier of infix operators
• [] can be used for several radix-related purposes when defining literals
• [] an empty boxy pair after an array is a Zen slice allowing adverbs and quoted string interpolation.
• [ ] after the name of an array while declaring it makes it a shaped array.

Operators containing this character

• [;] interpolates a list into a semicolon list, which is just a reduce meta operator but worth noting.
• [*-1] will appear often to find the last element of an array

When used inside regexes

• [] groups without capturing
• <[]> creates a character set

As an Adverb

• [] after an adverb can assign an Array object to an adverb but note that...
• [] after an adverb, if empty, is probably being used to specify a type of key for .k or .kv
• : [] used for extended identifiers when interpolation is desired

Hyper operator and meta operator restrictions

When used as the reduce meta-operator, only certain reduce operations have identity, others require multiple elements.

Old, deprecated, or other language uses

• [] is no longer a regular expression character set as in perl5. That is now <[]>
• [^] is no longer an inverted regex character set as in perl5. That is now <-> and <!>