Bach

An XML-interoperable general-purpose semantic document markup language

Bach is for anyone hand-authoring structured semantic text that is parsed into a data structure to be transformed programmatically. It is fully interoperable with your existing XML-based tooling, but it's a lot nicer to read and write!

This repository contains the language definition, a pure Python (3.4) parser as a reference implementation, a C parser (soon!) and test suite (soon!).

Installation

sudo pip3 install bach --upgrade --no-cache-dir

Key Features

Convert to and from XML and native language types

Use Bach with all the powerful tools you expect, like XLST, XQuery, DTD and XML Schemas.

Bach documents have a 1:1 mapping between several formats including XML, XHTML, Python literals, Python LXML Element Trees, "the subset of Lisp data where the first item in a list is required to be atomic", and more.

Shorthand attribute syntax configurable at parse-time

A special "shorthand" syntax can expand attributes like .myClass into class="myClass". The shorthand syntax is fully configurable at parse-time.

Efficient portable parser

Bach documents are parseable in linear time to the length of the input and one byte of lookahead i.e. with an LL(1) parser.

The language grammar is formally defined in a machine-readable document, then compiled into a compact portable representation. This makes it simpler and quicker to write a Bach parser in your chosen language.

Examples

Here is a basic example without XML namespaces, configured so that #foo #bar is expanded into tags="foo bar".

posts

    (post
        (title "Hello again")
        (content "Hello, this is my second post. Meow")
        #update #things #cats
    )

    (post
        (title "Hello world")
        (content
            (p "Hello, my website is"
                (a href="http://www.example.org/" "Example Website"))
            (img src="./example.png" "Example Image")
        )
        #personal #off-topic #wtf
    )

Here's an example with XML namespaces:

# Example based on https://msdn.microsoft.com/en-us/library/aa468565.aspx

d:student
    xmlns:d="https://www.example.org/student"
    xmlns:p='https://www.example.org/programming-language'

(d:id        "3235329")
(d:name      "Jeff Smith")
(p:languages "Python" "C" "Haskell")
(d:rating    "9.5")

Here's a contrived HTML4-style example:

html

(meta
    (title "Hello World!")
    (description "My Example Website!")
)

(div #header
    (a href="/"
        (img src="/logo.png" "Example.org Logo")
    )
)

(div #body
    (div.aside
        (p "Welcome to" (span.bold "Example Website!"))
    )
    (div.main
        (h1 #title "Hello World!")
    )
)

(div #foot
    (p "Copyright © Example Organisation")
)

Syntax and Semantics

A native Bach document is a 3-tuple of

• a label: like an XML tag name
• attributes: a mapping of attribute names to string values
• children: a mixed list of strings and subdocuments

In terms of syntax, a Bach document is a non-empty string that may start with #-style comments, followed by a label, then optionally attributes, shorthand attributes, string literals, and subdocuments in any order. Subdocuments may not contain #-style comments, but may contain #-style shorthand attributes instead.

Special characters are space, backslash, and

#=\t\r\n()[]{}<>"'

Special characters used for shorthand attributes are given to the parser as a mapping from shorthand symbols to attribute names.

A label is any string of non-special characters appearing as the first thing in a document or subdocument.

A string literal is quoted by single, double, or bracket quotes. Closing quotes may be escaped with backslash. A literal backlash must also be escaped.

'a string'
"a string"
[a string]
'a \' \\ string'
"a \" \\ string"
[a \] \\ string]

A shorthand attribute starts with a special shorthand character specified earlier.

A shorthand attribute then is followed by any string of non-special characters.

#anAttribute
.anAttribute
!anAttribute

This is quite similar to the syntax of CSS selectors. For example:

(anElement#someId.classOne.classTwo enabled title="An Example Element")

Full attributes are any string of non-special characters followed by an equals followed by a string literal. Whitespace is optional.

anAttribute="a value"
anAttribute = "a value"

The assignment may be ommitted, in which case the attribute is present but with an empty string value. This behaviour changed for XML compatibility

(item anAttributeWithEmptyValue)

Subdocuments start and end with brackets. They are always non-empty. String literals and subdocuments may always mix.

document (subdocument "a literal" (anotherSubdocument) "another literal")

Bach string literals and subdocuments follow XHTML-style whitespace normalisation rules for mixed content types (aka xs:complexType mixed="true", prevent this with xml:space="preserve"; you may like to configure Bach with a dedicated shorthand syntax to do this).

Open Standard

Tawesoft Ltd is committed to supporting Bach as an Open Standard. At this early stage we invite feedback and comments but, if and as soon as the need arises, we are keen to see democratic and inclusive stewardship of the language.

A full machine-readable formal grammar is given at grammar.txt. The rules are given in Greibach Normal Form with additional lookahead and semantic notation.

Name

Bach is named for the Welsh term of endearment (literally "small") and as tribute to the work of computer scientist Sheila Greibach.

Important Security Considerations

While Bach itself is not vulnerable to many common attacks on XML, such as exponential entity expansion (e.g. the "Billion laughs" attack), it is possible that a Bach document translated to XML may become a threat to an XML parser.

This reference implementation does not impose a limit on the length or complexity of a parsed Bach document. It is possible to create a Denial of Service (DoS) attack, or consume excessive processing time, if a server does not limit the rate of access to a Bach parser.

License

This license applies to the Bach Reference Implementation(s) and associated documentation.

Bach - XML-interoperable general-purpose semantic document markup language

Copyright © 2017 Ben Golightly <ben@tawesoft.co.uk>
Copyright © 2017 Tawesoft Ltd <opensource@tawesoft.co.uk>
Copyright © 2017 contributors; see CONTRIBUTORS.md

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