Sx

Sx is a framework, implemented in the Go programming language, to work with s-expressions (symbolic expressions). S-expressions are one of the most mature techniques to encode data in a portable and interpretable way. It follows a simple guide: every value is either an atomic value (like numbers, strings, symbols), or a list of values. A list might be empty. List are likely to be nested to allow more complex structures.

Many atomic values have an external representation. For example, the number zero is represented by the character sequence 0. A string is represented by a character sequence that is delimited by double quotes. A symbol is a sequence of non-space characters. A list is represented with a left, opening parenthesis, the possibly empty sequence of list elements, separated by a space, and a right, closing parenthesis.

S-expressions are also able to represent software functions. This is done by a simple trick: the first element of a list is interpreted as a function, all other elements are treated as arguments to this function. For example 3 + 4 can be encoded as a s-expression as (+ 3 4). This trick can easily transformed into a full calculus. There are several programming language based on s-epressions, notable LISP and Scheme.

Since s-expressions encode both data and source code, i.e. software, it is easy to write a software that works not just on ordinary data, but on software. To an extreme, a software might modify itself. A more moderate way is to write macros, or an internal domain specific language, which make you more productive.

Sx does not only provide raw s-expression, but also some functions to interpret s-expressions as software. This allows you to encode your data, but also to develop gigh-level software that works on that data. Sx provides an evaluator for s-expressions. Basically, it implements an LISP-1 interpreter, where functions and all other data use a single namespace. The programming language Scheme is also based on LISP-1. LISP itself, is a LISP-2, where functions reside in a different namespace.

This repository contains several sub-projects / packages:

• The package Sx itself contains all relevant data values, such as numbers, symbols, strings, …
• SxReader provides functions and data structures to read a s-expression representation and transforms it into the data values.
• SxEval provides a way to interpret s-expressions as software. This is done by evaluating s-expressions, resulting in a (possibly new) s-expression.
• SxBuiltins provide a collection of functions that can be used for a base evaluator.
• Package cmd contains a simple command line tool to work interactively with s-expressions. It combines all previous packages and also serves as an example to use them.

Use instructions

If you want to import this library into your own Go software, you must execute a go get command. Since Go treats non-standard software and non-standard platforms quite badly, you must use some non-standard commands.

First, you must install the version control system Fossil, which is a superior solution compared to Git, in too many use cases. It is just a single executable, nothing more. Make sure, it is in your search path for commands.

How you can execute the following Go command to retrieve a given version of this library:

GOVCS=t73f.de:fossil go get t73f.de/r/sx@HASH

where HASH is the hash value of the commit you want to use.

Go currently seems not to support software versions when the software is managed by Fossil. This explains the need for the hash value. However, this methods works.