Soul is an esoteric concatenative one-stack language, a small explorative language with an interpreter written in a day.

Most concatenative languages work on a state, often that's another stack. For example, Forth has a data and return stack and Postscript has an extra stack outputting to a page.

Soul is a minimalist language that provides one thing, and one thing only, the one sole stack. That, of course, is very problematic as a model of computation but Soul gets around that with two tricks:

1. Treat the stack as random access memory.
2. Whenever a constant integer or text is found on top of the stack, that gets swapped with whatever is underneath.

Soul expressions consist of integers, texts, and words. Words may define other expressions. A Soul expression defines a stack. The top of the stack is repeatedly evaluated until the stack is empty.

fetch, put, and delete inspect or modify the stack, they all take an integer argument where put takes a second argument to put somewhere in the stack.

The arithmetic operators +, -, *, / are all binary prefix operators. + is overloaded to work on texts also.

The equality operator = tests two arguments for equivalence, it results in either true or false. Both booleans are functions that select between two arguments; e.g., true f g evaluates to f.

print and line are the basic input/output words, to_int and to_text convert between integers and texts.

Hello World example:

"hello" print "world" print

Output 42, showcasing the defintion of a word:

# calculate the answer to everything
:f  * 6
+ 3 4 f print

The faculty function:

# the faculty function

# get
:get2   delete 1
:get1   fetch 5 + 3 delete get2
:get0   fetch 4 + 4 fetch get2 
:get    fetch 2 get0 get1 

# fac
:fac3   get 2 fac *
:fac2   fetch 3 - 1 fac3
:fac1   delete 1 1
:fac0   get 4 = 0 fac1 fac2
:fac    fetch 1 fac0 

fac 6 print

The Soul interpreter is written as an Egel script and implements a REPL.

You can run the interpreter with egel soul.eg. Either pipe the example programs to that process or copy-paste the source directly into the REPL.