[about document]
S-expr specification document[intended audience]
beginners in programming
S-expressions (Symbolic Expressions) are a fundamental concept in computer science and programming language theory. S-expressions are a simple, yet powerful notation for representing nested list data structures and code in a parenthesized form. They are commonly associated with the Lisp family of programming languages, where they serve both as a way to represent code and data uniformly.
Sexpression is a S-expression parsing library. Other than usual treatment of atoms and lists, it features peculiar decisions in syntax definition regarding strings and comments.
The general form of an S-expression is either:
atom), or(expr1 expr2 expr3)).Lists can be nested, allowing for the representation of complex hierarchical structures. For example:
(eq (mul x x) (pow x 2))
This S-expression depicts equality between multiplication and square.
The whitespace between syntactic elements may be used to improve readability, and may be consisted of spaces, new lines, or any combination of them.
Although a great part of S-expressions power lies in its simplicity, let’s introduce a few syntax extensions in a hope of making expressed code more readable, namely: strings and comments.
Strings in Sexpression may be single-line or multi-line. Single-line strings are atoms enclosed in "..." pairs, like in expression "this is a single-line string", and represent Unicode format strings. Multi-line strings are enclosed between an odd number greater than 1 of " symbols in the following manner:
"""
this is a
multi-line
string
"""
Enclosing between a pair of """ symbols, multi-line strings are bound in a rectangle enclosed between the start of the first """ symbol, and the end of the second """ symbol. We have to make sure that there are only space characters to the right of the first """ symbol and to the left of the second """ symbol. Remember to be careful when modifying contents of multi-line strings to make sure that the end of the second """ symbol is always placed horizontally behind the longest line in the string.
Notice that it is also possible to write expressions like:
(fst-atom """ trd-atom)
00001
00002
00003
"""
where the expression stands for three atoms in a list.
Comment expressions are ignored by the system, and they serve as notes to help programmers reading their code. They are parsed just like strings, only using the / instead of the " symbol. Thus, a single-line comment may be written as /this is a single-line comment/, and may appear repeatedly wherever a whitespace is expected. They can also be spanned over multiple lines, and an example of a multi-line comment may be:
///
this is a
multi-line
comment
///
Just like strings, enclosing between a pair of /// symbols, multi-line comments are bound in a rectangle between the start of the first /// symbol, and the end of the second /// symbol. We also have to make sure that there are only space characters to the right of the first /// symbol and to the left of the second /// symbol.
Notice that it is also possible to write expressions like:
///
00001 (
00002 fst-atom
00003 snd-atom
00004 trd-atom
00005 )
///
where the expression stands for three atoms in a list.
We defined Sexpression code format and introduced somewhat peculiar way to treat strings and comments. We tried to be consistent with these add-ons to keep acceptable ratio between simplicity and usability. The resulting code format is a bit more complicated than it is in usual Lispy languages, but we hope that the introduced complexity is justified by the data readability expressed this way.