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This project is part of the @thi.ng/umbrella monorepo.


Composable primitives for building declarative, transducer based Finite-State Machines & matchers for arbitrary data streams.

See the hiccup-markdown parser & example for a concrete use case.


DEPRECATED - superseded by other package(s)

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This package will be merged with and update the existing @thi.ng/transducers-fsm package.

Related packages


yarn add @thi.ng/fsm

ES module import:

<script type="module" src="https://cdn.skypack.dev/@thi.ng/fsm"></script>

Skypack documentation

For Node.js REPL:

const fsm = await import("@thi.ng/fsm");

Package sizes (brotli'd, pre-treeshake): ESM: 1.32 KB



Generated API docs

There're two key concepts provided by this package:


Matchers are composable functions which receive a single input value and attempt to match it to their configured criteria / patterns. Matchers also support optional user callbacks, which are executed when a match was made and are responsible for state transitions, state update and production of any result values.

See docs strings in /src folder for now.

FSM transducer

The fsm() function is a Finite-state machine transducer / iterator with support for single lookahead values. Takes an object of states and their matchers, an arbitrary context object and an initial state ID.

The returned transducer consumes inputs of type T and produces results of type R. The results are produced by callbacks of the given state matchers. Each can produce any number of values. If a callback returns a result wrapped w/ reduced(), the FSM causes early termination of the overall transducer pipeline. Failed state callbacks too can produce outputs, but will afterwards terminate the FSM.

An IllegalStateError will be thrown if a transition to an undefined state ID occurs.

The optional update function will be invoked for each input prior to executing the currently active state matcher. It is intended to update the context object (e.g. to update input location info for generating error messages).

If the optional src iterable is given, the function returns a transforming iterator of the FSM results.


If this project contributes to an academic publication, please cite it as:

  title = "@thi.ng/fsm",
  author = "Karsten Schmidt",
  note = "https://thi.ng/fsm",
  year = 2018


© 2018 - 2023 Karsten Schmidt // Apache License 2.0


npm i @thi.ng/fsm

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