Stem is a semantic templating engine.

What do I mean by a semantic templating engine? In most templating engines you usually write code something like this:

# Hi
% # This is a comment
% if user == "Bruno"
  {{user}} rhymes with Piano
% elsif user == "Brutus"
  {{user}} rhymes with Opus
% end

<?
  # Multiline code evaluation
  lucky = [1, 3, 7, 9, 13, 15]
  prime = [2, 3, 5, 7, 11, 13]
?>

{{ lucky & prime }}

In here the code to be placed inside the file is represented with strings, and strings are rather hard to work with. XML is a tree, and strings are one-dimensional arrays, not a perfect match. Stem instead uses scheme s-exprs (which are trees) to represent and generate xml code. As such it preserves the meaning of the xml and gives you full power to do any sort of code generation that you want. This is a short example:

# Hi
%{
(let loop ((table '()) (v '("hello" (b "hi") (i "sup"))) (row 1))
  (if (null? v) table
      (loop (append table (list `(tr (td ,row)
                                     (td ,row)
                                     (td ,(car v)))))
            (cdr v) (+ 1 row)))))
%}

Did I do a good job at making it sound pretentious? Well in reality this isn't really that much better than the snippet I showed you above. That's from mote which is a nice templating engine that's a whole (infurating) 10 lines shorter than stem, mostly because stem compiles sxml into xml while mote doesn't and that guile's regex search replace didn't work. Regardless I still think stem is easier to reason about but it's not really that much better.

Supported implementations. For now only guile, but I'll eventually make it R6RS compatible. The only part that isn't as of now is the regex part but that'll probably devolve into it's own project of making a fast implementation of SRFI 115 by using Ken Thompson's technique as described in Russ Cox's nice article

Usage. Don't, it's not worth it. But if you want to for whatever reason then you can figure it out yourself if you read the interface.scm file.