The following is the syntax of Sigil. Being a dialect of Lisp, it is spartan and simple, but and effective base for builing.

Numbers - a string of digits

Example:

1 is a number

100 is a number

3.141516 is a number

-3 is a number

Symbol - a string of characters not containing ’(’, or ’)’.

Example:

x is a symbol

abc is a symbol

x123 is a symbol

123x is a symbol

a-longer-symbol is a symbol

|a symbol with spaces| is a symbol

Comments - a string of characters up to a newline that is ignored

Example:

; this is a comment

Quoting - stops evaluation

Example:

1 =>;;;; 1

`1 =>;;;; 1

x =>;;;; ; Error, undefined symbol

`x =>;;;; x

(1 2 3) =>;;;; ; Error, undefined function

`(1 2 3) =>;;;; (1 2 3)

List - build lists

Example:

`() =>;;;; (); the empty list

`(1) =>;;;; (1) ; a ’proper’ list containing the digit 1

`(1 . ()) =>;;;; (1) ; a ’proper’ list containing the digit 1

`(1 2 3) =>;;;; (1 2 3) ; is a ’proper’ list of the digits 1 2 3

`(1 2 3 . ()) =>;;;; (1 2 3) ; is a ’proper’ list of the digits 1 2 3

`(1 . 2) =>;;;; (1 . 2) ; is an ’improper’ list of the digits 1 and 2

`(x . y) =>;;;; (x . y) ; is an ’improper’ list of the symbols x and y

A proper list has () as its last element, and it is not normally shown. An improper list does not have () as it’s last element, and this is shown by preceeding the last element with a dot.

Comma - evaulate inside of a quote

Example:

(set `x 1) =>;;;; 1

x =>;;;; 1

`(x 2 3) =>;;;; (x 2 3)

`(,x 2 3) =>;;;; (1 2 3)

Commaat - evaluate a list inside of a quote, and splice it

Example:

(set `x `(a b c))

x =>;;;; (a b c)

`(x 1 2 3) =>;;;; (x 1 2 3)

`(,x 1 2 3) =>;;;; ((a b c) 1 2 3)

`(,@x 1 2 3) =>;;;; (a b c 1 2 3)

NOTE: SYMBOLS AND NUMBERS ARE ALSO KNOWN AS ATOMS.

Booleans - () is false, everything else is true

As in most languages, Sigil has the idea of booleans, true and false. In this case, the empty list, () is ’false’ and everything else is ’true’. A special symbol ’t’ is defined to represent truth when needed, but is not required to be used.

Functions that are meant to return either a true or false value are known as ’predicates’.