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A Clojure library for sequencing events on a timeline.


timeline drawing

A timeline is a Clojure map which associates points in time (positions) with vectors of callbacks.

A callback (in this context) is a Clojure function with zero arguments.

As the sequencer is running, the current position advances through time. When we reach a position which has callbacks associated with it, the sequencer executes every callback in the associated vector.

The current position is incremented in ticks.

One tick is a predefined subdivision of a beat. With the default configuration, there are 96 ticks per beat and 120 beats per minute.


A pattern consists of four things:

  1. events: a vector of [offset callback] pairs
  2. snap: an alignment value expressed in ticks
  3. delay: a delay value expressed in ticks
  4. offset: the current offset of the pattern (only used while the pattern is built)

An offset is a position relative to the beginning of the pattern.

A pattern can be merged into the timeline at the current position. The merge operation adds the offset of each event plus the pattern delay to the current position of the timeline and injects the callbacks at the resulting absolute positions.

If snap is non-zero, the merge operation rounds up the current position of the timeline to the next multiple of snap and uses this position as the base when converting event offsets into absolute positions.

Pattern transformers

A pattern transformer is a function that transforms one pattern into another.

It has two arguments:

  1. the pattern to be transformed
  2. a map of bindings

Bindings are key-value pairs that can influence the operation of the pattern transformer.

Pattern expressions

A pattern expression is a data structure (a Clojure vector) which describes a pattern transformer. The type of the pattern transformer is identified by a keyword at the head of the vector. A pattern expression can be compiled into the corresponding pattern transformer via (sequencer/compile-pattern-expr).


Example: [:nop]

This is the identity transformer: it returns the pattern which was given to it.


Example: [:clear]

Returns an empty seed pattern:

{:events []
 :snap 0
 :delay 0
 :offset 0}


Example: [:call (fn [] (println "action!"))]

Pairs the current offset of the input pattern and the single argument (which must be a function) into an event and adds it to the events vector of the pattern.


Example: [:snap 4]

Sets the snap field of the pattern to the given number of beats (converted into ticks).


Example [:delay 1]

Sets the delay field of the pattern to the given number of ticks.


Example: [:wait 1]

  1. If the argument N is a positive number, adds N steps to the pattern offset
  2. If the argument N is a negative number, advances the pattern offset until it reaches the next multiple of -N steps

A step is a unit of duration expressed in beats.

The current step value is taken from the bindings (default: 1).


Example: [:seq [:bind {:step 2} [:wait 1]] [:bind {:step 3} [:wait 1]]]

Updates input bindings as described by the first map argument, then threads the input pattern through the given pattern transformers.

Each pattern transformer gets the updated bindings.

In the above example, the [:wait 1] pattern will wait 2 beats in the first bind and 3 beats in the second.

Binding values are not limited to constants, they can also be bind expressions:

  • [:add N]: adds N to the current binding value
  • [:sub N]: subtracts N from the current binding value
  • [:mul N]: multiplies the current binding value by N
  • [:div N]: divides the current binding value by N



  • [:var #'v]
  • [:var #'f 5]

Fetches the current binding of the var given in the first argument. If this value is a pattern transformer, applies it to the input pattern and bindings.

Otherwise it tries to convert the value into a pattern form. First it checks if the value is a Clojure function. If it is, applies it to the rest of the arguments inside the :var form and uses the return value as the pattern form. Otherwise uses the value as it is.

Finally it wraps the pattern form in a :bind expression - using the input bindings as its bind map -, compiles this :bind expression into a pattern transformer and applies the result to the input pattern and bindings.

The :var transformer can be used to support live coding.


Example: [:seq [:wait 2] [:call f] [:wait 3] [:call g]]

Threads the input pattern through the given pattern transformers.

Each pattern transformer gets the same bindings.


Example: [:mix [:call f] [[:wait 2] [:call g]] [:call h]]

Threads the input pattern through the given pattern transformers. Resets the pattern offset to its original value after each step.

This can be used to mix several patterns on top of each other.


Example: [:mix1 [:call f] [[:wait 2] [:call g]] [:call h]]

Same as [:seq [:mix [[:wait 2] [:call g]] [:call h]] [:call f]]

In other words, the first child form advances the offset but the others don't.


Example: [:play [:wait 8] [:var #'ornament]]

At compile-time, wraps the supplied patterns in a :seq and compiles the result.

At build-time, adds an event to the input pattern which builds (at run-time) a new pattern using the previously compiled transformer and the input bindings, then merges the resulting pattern onto the timeline.


Example: [:bpm 180]

Adds an event that changes the sequencer's BPM to the given value.

Pattern lifecycle

  1. START: Pattern exists as a Clojure data structure (pattern expression)
  2. COMPILE: Pattern expression is compiled into a pattern transformer
  3. BUILD: Pattern transformer is applied to seed pattern and initial bindings or the output of the previous step in a chain of pattern transformers built through functional composition
  4. MERGE: Output of pattern transformer is merged onto the timeline
  5. EXECUTE: Pattern events (callbacks) found on the timeline are executed by the sequencer

Syntactic sugar

Pattern forms

A pattern form is a Clojure value that can be translated into a pattern expression and then compiled into a pattern transformer.

By this definition the set of pattern forms includes all pattern expressions.

In addition, Clojure values matching the following predicates are automatically recognized as pattern forms and converted into the corresponding pattern expressions:

Clojure predicatePattern expression
var?[:var ...]
fn?[:call ...]
sequential?[:seq ...]
set?[:mix ...]

Hoisting of bind maps

The following form:

[:bind {:step 2}
  [:seq A B C]]

can be also written like this:

[:seq {:step 2} A B C]

Any maps found inside the [:seq ...] form are picked out, merged together left to right and then used as the bind map of an enclosing :bind form.

This also works for :mix and :mix1.


A target can extend the vocabulary of pattern forms and may also provide access to various software or hardware devices which can execute these new forms in specific contexts.

For example, the fluidsynth target manages a SoundFont MIDI synthesizer and provides forms like [:program N] to select a synthesizer patch and [:note N] to trigger a MIDI note.

Targets implement the following protocol:

  • (start t): start any devices associated with the target (e.g. an audio engine)
  • (stop t): stop the associated devices
  • (restart t): stop + start
  • (get-default-bindings t): return target-specific default bindings
  • (resolve-binding t k v): resolve a binding using target-specific logic
  • (compile-pattern-expr t pattern): extend the set of pattern expressions
  • (compile-pattern-form t form): extend the set of pattern forms
  • (compile-bind-expr t key expr): extend the set of bind expressions

To develop a new target, you need to implement the Target and TargetFactory protocols, then register the object which implements TargetFactory with the sequencer (sequencer/register-target-factory).

A target factory implements three methods:

  • (understands-descriptor? tf descriptor): returns true if the target factory can interpret the passed target descriptor
  • (sanitize-descriptor tf descriptor): converts the target descriptor into canonical form
  • (make-target tf descriptor): creates the actual target based on information in the descriptor

To use a target, you must first instantiate it with (sequencer/make-target descriptor), where descriptor is a data structure that describes the attributes of the desired target. The returned target shall be registered with the sequencer library (sequencer/register-target).

Once the target has been registered, you can bind it to the special :target key inside a bind form: this lets every pattern within the scope of that binding use the target's extensions.

Besides binding targets by value, you can also use target aliases. For this to work, you must pass two arguments to sequencer/register-target: the target itself and its alias. The alias must be a namespace-qualified keyword. Once a target has been registered with an alias, you can bind this alias to the :target key inside bind maps and it will be automatically resolved to the right target.

General usage

Create a sequencer object

(require '[omkamra.sequencer :as sequencer])

(def s (sequencer/create {:bpm 145 :tpb 24}))

The config map is optional:

(def s (sequencer/create))

In this case :bpm will default to 120 and :tpb to 96.

Play a pattern

(sequencer/play s [:seq [:wait 1] #(println "Hello, world!")])

This would start the sequencer and any previously registered targets (if they have not been started yet), compile the supplied pattern form, call the resulting pattern transformer with a seed pattern and initial bindings, then merge the resulting pattern into the timeline.

Seed pattern:

{:events []
 :snap 0
 :delay 0
 :offset 0}

Initial bindings (in case of the above example):

{:sequencer s
 :step 1}

You can add your own bindings by passing them as the third argument of sequencer/play.

Clear the timeline

(sequencer/clear! s)

This can be used to stop any unwanted mayhem.

Set the BPM of the sequencer

(sequencer/bpm! s 150)

Get the status of the sequencer

(sequencer/status s)

Start the sequencer and all registered targets

(sequencer/start s)

Stop the sequencer and all registered targets

(sequencer/stop s)

Restart the sequencer and all registered targets

(sequencer/restart s)


Copyright © 2021 Balázs Ruzsa

This program and the accompanying materials are made available under the terms of the Eclipse Public License 2.0 which is available at

This Source Code may also be made available under the following Secondary Licenses when the conditions for such availability set forth in the Eclipse Public License, v. 2.0 are satisfied: GNU General Public License as published by the Free Software Foundation, either version 2 of the License, or (at your option) any later version, with the GNU Classpath Exception which is available at

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