package note

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Declarative events and signals for OCaml


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Note — Declarative events and signals for OCaml


Note is an OCaml library for functional reactive programming (FRP). It provides support to program with time varying values: declarative events and signals.

Note also has (optional and experimental) support for reactive browser programming with the brr library.

Note is distributed under the ISC license.



Note can be installed with opam:

opam install note
opam install note brr  # For the browser support

If you don't use opam consult the opam file for build instructions.


The documentation can be consulted online or via odig doc note.

Questions are welcome but better asked on the OCaml forum than on the issue tracker.

Sample programs

An implementation of the TodoMVC application with note.brr is in

You can run it with b0 -- todomvc see also b0 list for other tests to run.


Note is a potential successor to the OCaml React library.

On the plus side:

  • Uses a simpler push/pull implementation which does not uses weak references. Combinators are easier to implement and understand.

  • Provides a formal API to interface the outputs of the reactive system rather than rely on effectful events and signals. Enforces life-time management of the output of the reactive system and could provides (dynamic) detection of constant signals and never occuring events.

  • Provides (hopefully) a better combinator set. Especially with respect to the pain point of signal initialization in React: in Note, due the pull based strategy, {E,S}.value is safe and sound to use.

On the minus side:

  • The life-time of the outputs of the reactive system have to be explicitely managed; but we argue this has to be done anyways in practice especially in browsers due to lack of weak references.

  • It is easier for code interfacing the outputs of the reactive system to break the FRP denotational semantics and thus equational reasoning. However the discipline needed not to do so is clear and simple: do not reobserve a signal/event that was no longer observed.

  • The depth first DAG update strategy of Note may be subject to stackoverflows on deep DAGs. We suspect however that this should not be a problem in practice.

On the unknown side:

  • Memory footprint is likely to be smaller in Note. Nodes of the DAG do not keep track of the nodes that depend on them via weak references. They do keep track of the root nodes of the DAG they depend on, but these sets can be shared among nodes.

  • Lack of weak references in Note may improve performance.

  • On updates the number of nodes that have to be visited (not recomputed) is larger in Note. In React this is the minimal number of nodes effectively affected by the update, in Note this is all the nodes thay may be affected by the update. However react also needs a priority queue with weak references for its update, Note does not need this and allows to update the graph at any point where it might be needed. The latter brings API usability improvements, e.g. the sound and safe implementation of {E,S}.value in Note.


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