So far you have dealt with atoms that are either numeric or character. The monad < (box) introduces a new type of atom called boxed. The monad < applies to any noun and returns an atom that is a box which contains the argument.
An array is either a numeric array that contains numbers, or a literal array that contains characters, or a boxed array that contains boxes. Arrays of numbers and characters are referred to as open to distinguish them from boxed arrays.
b =. < 2 3 4 $b NB. an atom has empty shape
A boxed array is displayed in a box.
b +-----+ ¦2 3 4¦ +-----+ c =. < 4 7 9 d =. b , c NB. append d +-----------+ ¦2 3 4¦4 7 9¦ +-----------+ $d NB. list with shape 2 2 (< 2 3 4) , < 4 7 9 +-----------+ ¦2 3 4¦4 7 9¦ +-----------+
Arrays of different types (numeric, character, and boxed) cannot be appended to one another.
'a' , 3 ¦domain error ¦ 'a' ,3 3 , < 2 3 4 ¦domain error ¦ 3 ,<2 3 4
Boxed arrays are of the same type and can be appended no matter what they contain.
(< 'abc') , < 4 5 6 +---------+ ¦abc¦4 5 6¦ +---------+ (< 2 3 $ 'abcdef') , (< i. 3 4) , < 23 +----------------+ ¦abc¦0 1 2 3¦23¦ ¦def¦4 5 6 7¦ ¦ ¦ ¦8 9 10 11¦ ¦ +----------------+