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.

Usage instructions

To import this library into your own Go software, you need to run the go get command. Since Go does not handle non-standard software and platforms well, some additional steps are required.

First, install the version control system Fossil, which is a superior alternative to Git in many use cases. Fossil is just a single executable, nothing more. Make sure it is included in your system's command search path.

Then, run the following Go command to retrieve a specific version of this library:

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

Here, HASH represents the commit hash of the version you want to use.

Go currently does not seem to support software versioning for projects managed by Fossil. This is why the hash value is required. However, this method works reliably.