An atom node consists of a single node identifier, referred to as a symbol. Node identifiers may consist of any sequence of characters excluding whitespace, line breaks, and the following special characters: (, ), {, }, ", ,, ., ;, #. Node identifiers must consist of at least one non-digit character, otherwise they are interpreted as numbers.

The following are all examples of valid node identifiers:

The following are not valid node identifiers:

A functor node is an expression consisting of an operator node applied to zero or more argument nodes. Syntactically the operator node is written first followed by the comma-separated list of argument nodes in parenthesis (...).

Examples. 

op(arg)
func(arg1, arg2)
m.fn()

	A line break occurring before the closing parenthesis ) is not treated as a declaration separator. Instead the following line is treated as a continuation of the current line.

In the special case of an operator applied to two arguments, the operator may be placed in infix position, that is between the two argument nodes. The operator may only be an atom node and must be registered as an infix operator.

	Spaces between the infix operator and its operands are required, in order to distinguish the operator from the operands. This is due to there being few restrictions on the characters allowed in node identifiers.

	A line break occurring between the infix operator and its right argument is not treated as a declaration separator. However a line break between the left argument and infix operator is treated as terminating the declaration consisting of the left argument. The following line is then treated as a new separate declaration.

Functor expressions written in infix and prefix form are equivalent, thus the following infix functor expression:

a + b

is equivalent to the following prefix functor expression (either expression may be written in source code):

+(a, b)

Each node registered as an infix operator has, associated with it, a precedence and associativity. The precedence is a number that controls the priority with which the operator consumes its arguments. Operators with a higher precedence consume arguments before operators with a lower precedence. The associativity controls whether the operands are grouped starting from the left or right, in an expression containing multiple instances of the same infix operator.

a + b * c

The * operator has a greater precedence than the + operator, thus it consumes its arguments first, consuming the b and c arguments.

The + operator has a lower precedence thus it consumes its arguments after *. The arguments available to it are a and *(b, c).

As a result the infix functor expression is parsed to the following:

+(a, *(b, c))

Use parenthesis to control which arguments are grouped with which operators. Thus for an infix expression to be parsed to the following, , assuming the * operator has a greater precedence than +:

*(+(a, b), c)

a + b must be surrounded in parenthesis:

(a + b) * c

Multiple declarations can be syntactically grouped into a single node expression using the { and } delimiters. All declarations between the delimiters are processed and accumulated into a special node list syntactic entity. In most contexts the node list is treated as being equivalent to the last node expression before the closing brace }, however special operators and macros may process node lists in a different manner.

Node Lists. 

<node> = <node list>
<node list> = '{' <declaration>* '}'

	Each node list must be terminated by a closing brace } further on in the file otherwise a parse error is triggered.

Literal nodes include numbers and strings.

Numbers

There are two types of numbers: integers and real-valued numbers, referred to as reals, which are represented as floating-point numbers.

Integers consist of a sequence of digits in the range 0—9, optionally preceded by the integer’s sign. A preceding - indicates a negative number. A preceding + indicates a positive integer, which is the default if the sign is omitted.

Integer Syntax. 

<integer> = ['+'|'-']('0'..'9')+

There are numerous syntaxes for real-valued numbers. The most basic is the decimal syntax which comprises an integer followed by the decimal dot . character and a sequence of digits in the range 0—9.

Decimal Real Syntax. 

<real> = <decimal> | <magnitude-exponent>

<decimal> = <integer>'.'('0'..'9')+

	The decimal . must be preceded and followed by at-least one digit character. Thus .5 and 1. are not valid real literals, 0.5 and 1.0 have to be written instead.

The exponent syntax allows a real-number to be specified in scientific notation as a magnitude and exponent pair . The exponent syntax comprises a real in decimal syntax or an integer, followed by the character e, f, d, or l which indicates the precision of the real-number, followed by the exponent as an integer.

Exponent Syntax. 

<magnitude-exponent> = (<decimal>|<integer>)['e'|'f'|'d'|'l']<integer>

e and f indicate a single precision floating point number, d indicates double precision and l indicates long precision.

Strings

Literal strings consist of a sequence of characters enclosed in double quotes "...".

String Syntax. 

<string> = '"'<unicode char>*'"'

where <unicode char> can be any Unicode character.

A literal " character can appear inside a string if it is preceded by the backslash escape character \.

Example. 

"John said \"Hello\""

Certain escape sequence, consisting of a \ followed by a character, are shorthands for special characters, allowing the character to appear in the parsed string without having to write the actual character in the string literal.

Table 1. Escape Sequences

Sequence	Character	ASCII Character Code (Hex)
\n	Line Feed (LF) / New Line	0A
\r	Carriage Return (CR)	0D
\t	Tab	09
\u{<code>}	Unicode Character	<code>

The \u{<code>} escape sequence is replaced with the Unicode character with code (in hexadecimal) <code>. There must be an opening brace { following \u otherwise the escape sequence is treated as an ordinary literal character escape, in which \u is replaced with u. Currently the closing brace is optional }, as only the characters up to the first character that is not a hexadecimal digit are considered part of the character code. However, it is good practice to insert the closing brace as it clearly delimits which characters are to be interpreted as the character code and which characters are literal characters.

	The \n, \r and \t escape sequences can alternatively be written as \u{A}, \u{D} and \u{9} respectively.

	In a future release, omitting either the opening or closing brace, in a Unicode escape sequence, may result in a parse error.