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


Functional & composable primitives for building declarative, transducer based Finite-State machines and parsers for arbitrary input data streams (not necessarily string based).

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


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


yarn add @thi.ng/fsm


Usage examples

Checkout the minimal Markdown parser example:

Source code | Live demo

The parser itself is defined here: @thi.ng/hiccup-markdown

import * as fsm from "@thi.ng/fsm";


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.


  • Karsten Schmidt


© 2018 Karsten Schmidt // Apache Software License 2.0

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