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:musical_score: Semantic music notation

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Bach logo



bach is a semantic music notation designed to be both human and computer friendly.

The project is pre-alpha and is not should not be considered stable for production use.


  • Native support for semantic music constructs such as chords and scales
  • Allow for alternative real-time representations of music (e.g. visual instead of just audio)
  • Seamless synchronization with associated audio data by minimizing the complexities around timing
  • Easy to read for both humans and computers
  • Easy to translate from sheet music
  • Small learning curve
  • Highly productive
  • Trivial to interpret compiled output. Writing bach engines should be easy!
  • Keeps your definitions DRY


bach tracks are ultimately interpreted by a higher-level bach engine, such as gig.

This module, by itself, can only parse and compile plaintext bach data into bach.json.

bach.json is a JSON micro-format that makes it trivial for bach engines to sequentially process a bach music track and synchronize it in real-time with audio.


The following bach track represents the scale progression of a blues song:

@Audio = ''
@Title = 'Jimi Style 12-Bar-Blues Backing Track in A'
@Instrument = 'guitar'
@Time = 4|4
@Tempo = 42

:A = Scale('A3 minorpentatonic')
:D = Scale('D3 minorpentatonic')
:E = Scale('E3 minorpentatonic')

:Track = [
  1 -> :A
  1 -> :D
  2 -> :A
  2 -> :D
  2 -> :A
  1 -> :E
  1 -> :D
  2 -> :A

!Play :Track

and is interpreted like so:

  1. Scale :A, or A3 minorpentatonic, will be played for 1 measure, then
  2. Scale :D, or D3 minorpentatonic, will be played for 1 measure, then
  3. Scale :A will be played for 2 measures, then
  4. ...

To find a list of every construct supported by bach (such as Note, Chord, etc.), please refer to the "Constructs" section.



[bach "1.0.0"]


compile "bach:bach:1.0.0"




$ lein install


$ lein test



First be sure that you have a binary executable (requires lein to be installed) available on your PATH:

$ lein bin

Then you can execute the resulting binary like so:

$ target/bach -i /path/to/track.bach compile

The executable currently supports the following actions:

  • parse: creates an Abstract Syntax Tree (AST) from vanilla bach data
  • compile: parses and compiles vanilla bach data into bach.json, an intermediary JSON micro-format that allows for simple interpretation of tracks
  • help


(ns my.namespace
  (:require [bach.ast :refer [parse]]
            [bach.track :refer [compile-track]]))

; parses and compiles raw bach data into an interpretable hash-map
(compile-track (parse "!Play [1 -> Chord('A'), 1 -> Chord('C')]"))


An Extended Backus-Naur Form (EBNF) formatted definition of the grammar can be found in grammar.bnf.


Beats represent either an Element or a Collection of Elements that are played for a duration of time.

Elements are either Chords, Scales, Notes, Rests (~), or Collections.

The duration that a Beat is played for is specified using the tuple symbol, ->:

<duration> -> <element>


Beats defined in Lists will be played sequentially in the natural order (left to right) and will not overlap.

[<duration> -> <element>, <duration> -> <element>]


Beats defined in Sets will be played in parallel and may overlap.

{<duration> -> <element>, <duration> -> <element>}


Sets or Lists defined in other Sets (i.e. nested Sets) may be defined without a duration.

    <duration> -> <element>
    <duration> -> <element>
    <duration> -> <element>
    <duration> -> <element>


The value of a Beat's <duration> can be:

1    = Whole note (or one entire measure in 4|4)
1/2  = Half note
1/4  = Quarter note
1/8  = Eighth note
1/16 = Sixteenth note
1/512 = Minimum duration

To adhere with music theory, durations are strictly based on common time (4|4).

This means that 1 always means 4 quarter notes, and only equates with a full measure when the number of beats in a measure is 4 (as in 4|4, 3|4, 5|4, etc.).

The examples in the remainder of this section assume common time, since this is the default when a @Time header is not provided.


A List playing a Note('C2') for an entire measure, starting at the first Beat, would be specified like so:

[1 -> Note('C2')]

If you wanted to start playing the note on the second Beat of the measure, then simply rest (~) on the first Beat:

[1/4 -> ~, 1 -> Note('C2')]

When a Beat tuple is not provided in an an assignment or a Collection, both the position and duration of the Beat will be implied at run-time to be the index of each respective element as they are played.

The position and duration are both determined by the time signature (the default is common time, or 4|4).

For instance:

[1/4 -> Note('C2'), 1/4 -> Note('F2')]

is the same as:

[Note('C2'), Note('F2')]

Beat durations can also use basic mathematical operators. This makes the translation between sheet music and bach an easy task.

1 + 1/2 -> Chord'(C2min6')

This is usefeul for specifying more complicated rhythms, like those seen in jazz.

:Mutliple = [
  1/2   -> Chord('D2min7')
  1+1/2 -> Chord('E2maj7')
  1+1/2 -> Chord('C2maj7')

You may also use the -, * and / operators.


All Elements, unless already nested in a List or Set, must be instantiated in a Beat tuple (or implicitly converted into one, as shown in the previous section).

The first parameter of every Element is a string formatted in scientific pitch notation (SPN) (surrounded with ' or ") such as 'C2', which is a second octave C note.


As a convenience, Elements may also be defined implicitly, specified using a #:

:Note  = #('C2')
:Chord = #('C2Maj7')
:Scale = #('C2 Minor')

Determining the semantic value of implicit Elements (i.e. whether it's a Note, Chord, etc.) is the responsibility of the bach interpreter.

It's suggested that you primarily use implicits as they will save you a lot of typing over time.


To assign a variable, prefix a unique name with the : operator and provide a value (<element>):

:MyLoop = [1 -> Note('C2'), 1 -> Note('E2')]

Once assigned a name, variables may be dynamically referenced anywhere else in the track:

:CoolLooop = :MyLoop


In music it's common to see cadence sections labeled as A, B, C, and so on. bach's syntax favors this nicely:

:A = Chord('F2maj')
:B = Chord('G2maj')
:C = Chord('C2maj')

:Song = [
  1 -> :A
  1 -> :B
  1 -> :C
  1 -> :A

!Play :Song

Destructured list assignments will soon be supported and will also favor cadences (currently unsupported):

:[A, B, C, D] = [Chord('E7'), Chord('Emin7'), Chord('Cmaj7'), Chord('Dmaj7')]


Arbitrary attributes may be associated with Elements using the <key>: <value> syntax. These attributes allow you to cusotmize the representations and interpretations of your Elements.

For instance, colors are useful for succesfully expressing a variety of data to the user at once. You might also want to specify the specific voicing of a chord.

:ABC = [
  1 -> {
    Scale('C2min',  color: #6CB359)
    Chord('D2min7', color: #AA5585, voicing: 1)
  1 -> Chord('G2maj7', color: #D48B6A, voicing: 2)
  2 -> Chord('C2maj7', color: #FFDCAA, voicing: 2)


Optional header information, including the tempo and time signature, is specified with assignments at the top of the file and prefixed with the @ operator:

Headers outside of those defined in the documentation are allowed and can be interpreted freely by the end user, just like X- headers in HTTP. The value of custom headers can be of any primitive type.

@Title  = 'My bach track'
@Time   = 4|4
@Tempo  = 90
@Tags   = ['test', 'lullaby']
@Custom = 'so special'

:ABC = [
  1/2 -> Chord('D2min7')
  1/2 -> Chord('G2min7')
  1 -> Chord('C2maj7')


Because bach supports references, it requires a mechanism for specifying which data should be used for playing the track. You can think of Play as your main method or default export.

In other words, you need to tell it which values should ultimately be made available to the bach interpreter.

Any Elements that aren't being referenced or used by the value exported with !Play will be ignored during compilation.

:Ignored  = [1 -> Chord('D2min6'), 1 -> Chord('A2min9')]
:Utilized = [1 -> Chord('C2Maj7'), 1 -> Chord('A2Maj7')]

!Play :Utilized

Only one !Play definition is allowed per track file.



  • Note = Single note in scientific notation
  • Scale = Scale in scientific notation
  • Chord = Chord in scientific notation
  • Mode = Mode in scientific notation
  • Triad = Triad of notes in scientific notation
  • ~ = Rest
  • # = Implicit (the interpreter determines if it's scale, chord or note based on the notation itself)
  • [] = List (sequential / ordered)
  • {} = Set (parallel / unordered)


  • Tempo (integer, beats per minute)
  • Time (meter, time signature. ex: 6|8, 4|4)
  • Key (string, key signature)
  • Audio (url)
  • Instrument (string, arbitrary)
  • Title (string, arbitrary)
  • Artist (string, arbitrary)
  • Desc (string, arbitrary)
  • Tags (list or set of strings, arbitrary)
  • Link (string, url)


  • + = Add
  • - = Subtract
  • / = Divide
  • * = Multiply
  • | = Meter (for time signatures, not arbitrary mathematical expressions)


  • 'foo' or "bar" = string
  • 123 or 4.5 = number
  • #000000 = color


  • bach-json-schema contains the official JSON Schema definition for the bach.json format
  • bach-rest-api is a RESTful HTTP service that allows compilation of bach tracks into bach.json
  • gig is the official NodeJS bach interpreter library


If you encounter any problems or have any questions then please feel free to open up an issue.


Contributions are always welcome. Simply fork, make your changes and then create a pull request with thorough tests.


  • [ ] Write technical specfiication
  • [X] General work towards making the tracks iterable in a normalized fashion
  • [ ] Refactor headers (rename to "metas") so that they are contextual instead of global
  • [ ] Create @Dynamics contextual meta construct (alternatives: @Intensity, @Volume)
  • [ ] Create @Accent contextual meta construct
  • [ ] Replace !Play with more generic export keyword
  • [ ] Element relationships (e.g. tie, slur, glissando, arpeggiated chord)
  • [ ] Tuplets
  • [ ] Destructured list assignments
  • [ ] Expandable / destructured scales and chords (i.e. [... 1/4 -> Scale('C2')])
  • [ ] Application of collection variables (i.e. dereference and flatten value into surrounding list)
  • [ ] Allow user to define sections of a track that should loop forever
  • [ ] Allow track linking with Hypermedia
  • [ ] Linkable sections with unique namespaces so that end users may bookmark and/or track progress, or specify areas to loop
  • [X] Hide Chord or Scale (so it's only functionally relevant and not highlighted to the user)
  • [x] Arbitrary classification of notes (i.e. Note('C2', class: "blue"))
  • [x] Chord voicings/inversions (i.e. Chord('C2maj7', inversion: 1))
  • [x] Traids (root, 1st, 2nd)
  • [ ] Sort out complexities around Beats in nested Lists (i.e. should nested Beats / Elements be able to surpass their parents?)

Can you improve this documentation? These fine people already did:
evavro & Erik Vavro
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