### Characters

[
  Type     characters                          Alias chars
  Datatype unicode-characters                  Alias unicode-chars
  Type     unicode-points
  Funcon   unicode-character                   Alias unicode-char
  Funcon   unicode-point                       Alias unicode
  Type     basic-multilingual-plane-characters Alias bmp-chars
  Type     basic-multilingual-plane-points
  Type     iso-latin-1-characters              Alias latin-1-chars
  Type     iso-latin-1-points
  Type     ascii-characters                    Alias ascii-chars
  Type     ascii-points
  Funcon   ascii-character                     Alias ascii-char
  Funcon   utf-8
  Funcon   utf-16
  Funcon   utf-32
  Funcon   backspace
  Funcon   horizontal-tab
  Funcon   line-feed
  Funcon   form-feed
  Funcon   carriage-return
  Funcon   double-quote
  Funcon   single-quote
  Funcon   backslash
]


Built-in Type
  characters <: values
/*
  Literal characters can be written `'C'` where `C` is any visible character
  other than a `single-quote` or `backslash` character, which need to be
  escaped as `'\''` and `'\\'`.
*/
Alias
  chars = characters


#### Unicode character set
/*
  The set of Unicode characters and allocated points is open to extension.
  See https://en.wikipedia.org/wiki/Plane_(Unicode)
*/

Built-in Datatype
  unicode-characters <: characters
Alias
  unicode-chars = unicode-characters
Built-in Type
  unicode-points <: bounded-integers(0, unsigned-bit-vector-maximum(21))
Built-in Funcon
  unicode-character(_:unicode-points) : unicode-characters
Alias
  unicode-char = unicode-character
/*
  The values in `unicode-characters` are the values of
  `unicode-character(UP:unicode-points)`.
*/
Funcon
  unicode-point(_:unicode-characters) : =>unicode-points
Alias
  unicode = unicode-point
Rule
  unicode-point(unicode-character(UP:unicode-points)) ~> UP


#### Unicode basic multilingual plane
/*
  The set of Unicode BMP characters and allocated points is open to extension.
*/

Built-in Datatype
  basic-multilingual-plane-characters <: unicode-characters
Alias
  bmp-chars = basic-multilingual-plane-characters
Built-in Type
  basic-multilingual-plane-points <:
    bounded-integers(0, unsigned-bit-vector-maximum(17))
/*
  The values in `basic-multilingual-plane-characters` are the values of
  `unicode-character(BMPP:basic-multilingual-plane-points)`.
*/


#### ISO Latin-1 character set

Built-in Datatype
  iso-latin-1-characters <: basic-multilingual-plane-characters
Alias
  latin-1-chars = iso-latin-1-characters
Type
  iso-latin-1-points ~> bounded-integers(0, unsigned-bit-vector-maximum(8))
/*
  The values in `iso-latin-1-characters` are the values of
  `unicode-character(ILP:iso-latin-1-points)`.
*/


#### ASCII character set

Built-in Type
  ascii-characters <: iso-latin-1-characters
Alias
  ascii-chars = ascii-characters
Type
  ascii-points ~> bounded-integers(0, unsigned-bit-vector-maximum(7))
/*
  The values in `ascii-characters` are the values of
  `unicode-character(AP:ascii-points)`.
*/
Funcon
  ascii-character(_:strings) : =>ascii-characters?
Alias
  ascii-char = ascii-character
/*
  `ascii-character"C"` takes a string. When it consists of a single ASCII
  character `C` it gives the character, otherwise `( )`.
*/
Rule
  ascii-character[C:ascii-characters] ~> C
Rule
  C : ~ ascii-characters
  ------------------------------------
  ascii-character[C:characters] ~> ( )
Rule
  length(C*) =/= 1
  --------------------------------------
  ascii-character[C*:characters*] ~> ( )


#### Character point encodings
/*
  See https://en.wikipedia.org/wiki/Character_encoding
*/

Built-in Funcon
  utf-8(_:unicode-points) : =>(bytes, (bytes, (bytes, bytes?)? )? )
Built-in Funcon
  utf-16(_:unicode-points) : =>(bit-vectors(16), (bit-vectors(16))? )
Built-in Funcon
  utf-32(_:unicode-points) : =>bit-vectors(32)


#### Control characters

Funcon
  backspace : =>ascii-characters 
    ~> unicode-character(hexadecimal-natural"0008")
Funcon
  horizontal-tab : =>ascii-characters 
    ~> unicode-character(hexadecimal-natural"0009")
Funcon
  line-feed : =>ascii-characters 
    ~> unicode-character(hexadecimal-natural"000a")
Funcon
  form-feed : =>ascii-characters 
    ~> unicode-character(hexadecimal-natural"000c")
Funcon
  carriage-return : =>ascii-characters 
    ~> unicode-character(hexadecimal-natural"000d")
Funcon
  double-quote : =>ascii-characters 
    ~> unicode-character(hexadecimal-natural"0022")
Funcon
  single-quote : =>ascii-characters 
    ~> unicode-character(hexadecimal-natural"0027")
Funcon
  backslash : =>ascii-characters 
    ~> unicode-character(hexadecimal-natural"005c")