Example how bash function input/output can be used to compose

xmlgen.sh

 #!/bin/bash

# Creates an XML element from the first parameter. Remaining parameters and all stdin are copied to stdout.
function elem() {
    local name=$1
    echo -n "<$name>"
    shift
    echo -n $*
    [[ ! -t 0 ]] && cat
    echo "</$name>"
}

function title() {
    elem "title" $*
}
function author() {
    elem "author" $*
}
function book() {
    elem "book" $*
}
function library() {
    elem "library" $*
}

{
    {
        title "Moby Dick"
        author "Melville"
    } | book
    {
        title "Das Capital"
        author "Marx"
    } | book
    {
        title "The Cat In The Hat"
        author "Suess"
    } | book
} | library

I guess the Python version would look like this:

def elem(name, content=[]):
    return ["<%s>" % name] + content + ["</%s>" % name]

def title(content):
    return elem("title", content)

def author(content):
    return elem("author", content)

def book(content):
    return elem("book", content)

def library(content):
    return elem("library", content)

print "".join(
        library(
            book(
                title(["Moby Dick"]) +
                author(["Melville"])
            )
            + book(
                title(["Das Capital"]) +
                author(["Marx"])
            )
            + book(
                title(["The Cat In The Hat"]) +
                author(["Suess"])
            )
        )
    )

Simpler but less DSLish, I guess.

Still very readable - I like it.