====== Literals ======

RL provides the following literals.

  * Symbols
  * Strings
  * Numbers
  * Bit Strings

Note that [[rl:termliterals|term literals]] are described elsewhere.

An odd property of the literals is that, while each has a default type, you can //override// the type if you wish.

===== Symbols =====

A **symbol** has two forms.

  * A "usual" symbol starts with either a letter or an underscore followed by letters, underscores, or digits, with the exception that the lone underscore is not permitted to avoid conflict with the wildcard (see [[rl:primitives|Primitives]]).
  * A "quoted" symbol is any sequence of characters enclosed in single quotation marks (''%%'..'%%''), including the special escape sequences ''\n'' (newline), ''\r'' (carriage return), ''\t'' (tab), ''\\'' (backslash), ''\"'' (double quotation mark), ''%%\'%%'' (single quotation mark), and ''\\u{N}'' where ''N'' is a hexadecimal number of up to six characters specifying a Unicode code point.

While you don't really need to escape double quotation marks in a symbol, the choice to allow the escape makes this consistent with strings.
 
The following are examples of symbols.

    fred
    _primary_
    ''
    '5'
    '$ 9\'-'
    Set_3

The default type for all symbols is the root type ''SYMBOL'', but the type of a symbol can be given explicitly.  This allows distinguishing between root types (like ''STRING:^ROOT'') and symbols with the same name (like ''STRING:SYMBOL'').  The following are legal symbols.

|''%%'_'%%'' |Would otherwise conflict with the wildcard '_' |
|''%%'^ROOT'%%'' |Would otherwise conflict with the root term |
|''%%'ANY'%%'' and ''ANY:SYMBOL'' |Would otherwise conflict with the wildcard 'ANY' |
|''%%'NONE'%%'' and ''NONE:SYMBOL'' |Would otherwise conflict with the wildcard 'NONE' |
|''%%'INTEGER'%%'' and ''INTEGER:SYMBOL'' |Would otherwise conflict with the root type 'INTEGER' |

<ebnf> {
symbol = usual_symbol | quoted_symbol .
usual_symbol = "[a-zA-Z][a-zA-Z0-9_]*" | "_[a-zA-Z0-9_]+" .
quoted_symbol = "'" { '[^\\\\"]' | ESCAPE } "'" .
ESCAPE = '\\\\n' | '\\\\r' | '\\\\t' | '\\\\\\\\' | '\\\\"' | "\\\\'" | UNICODE .
UNICODE = '\\\\u{' "[0-9a-fA-F]+" '}' .
} </ebnf>

===== Strings =====

A **string** has two forms.

  * A "usual" string is any sequence of characters enclosed in double quotation marks (''%%".."%%''), including the special escape sequences ''\n'' (newline), ''\r'' (carriage return), ''\t'' (tab), ''\\'' (backslash), ''\"'' (double quotation mark), ''%%\'%%'' (single quotation mark), and ''\\u{N}'' where ''N'' is a hexadecimal number of up to six characters specifying a Unicode code point.
  * A "long" string is any sequence of characters enclosed in triple quotation marks (''""".."""'').  Character escapes are not interpreted; all characters are preserved as-is.

As we will see later, strings can be composed with the //applicative dot//, so ''"Hello "."""world"""'' becomes the string ''"Hello world"''.