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Circlet is an HTTP and networking library for the janet language. It provides an abstraction out of the box like Clojure's ring, which is a server abstraction that makes it easy to build composable web applications.

Circlet uses mongoose as the underlying HTTP server engine. Mongoose is a portable, low footprint, event based server library. The flexible build system requirements of mongoose make it very easy to embed in other C programs and libraries.


You can add Circlet as a dependency in your project.janet:

  :name "web framework" :description "A framework for web development"
  :dependencies [""])

You can also install it system-wide with jpm:

sh jpm install circlet


Creating a server

You can create a HTTP server using the circlet/server function. The function is of the form (circlet/server handler port &opt ip-address) and takes the following parameters:

  • handler function that takes the incoming HTTP request object (explained in greater detail below) and returns the HTTP response object.
  • port number of the port on which the server will listen for incoming requests.
  • ip-address optional string representing the IP address on which the server will listen (defaults to “”). The address “*” will cause the server to listen on all available IP addresses.

The server runs immediately after creation.


The handler function takes a single parameter representing the request. The request is a Janet table containing all the information about the request. It contains the following keys:

  • :uri requested URI
  • :method HTTP method of the request as a Janet string (e.g. "GET", "POST")
  • :protocol version of the HTTP protocol used for request
  • :headers HTTP headers sent with the request as a Janet table. Keys in this table are Janet strings with standard header's name (e.g. "Host", “Accept"). Values are the values in the HTTP header.
  • :body body of the HTTP request
  • :query-string query string part of the requested URI
  • :connection internal mongoose connection serving this request


The return value of the handler function must be a Janet table containing at least the status key with an integer value that corresponds to the HTTP status of the response (e.g. 200 for success).

Other possible keys include:

  • :body the body of the HTTP response (e.g. a string in HTML or JSON)
  • :headers a Janet table or struct with standard HTTP headers. The structure is the same as the HTTP request case described above.

There is also special key :kind you can use. There are two possible values for this key:

  • :file for serving a file from the filesystem. The filename is specified by the :file key. You can specify :mime key with value of corresponding mime type, it defaults to text/html.
  • :static for serving static file from the filesystem. You have to provide :root key with value of the path you want to serve.


Circlet also allows for the creation of different “middleware”. Pieces of middleware can be thought of as links in a chain of functions that are used to consume the HTTP request. The handler function can be thought of as a piece of middleware and other middleware should match the signature and return type of the handler function, i.e. accept and return a Janet table.

Middleware can be created in one of two ways. A user can define a function with the appropriate signature and return type or use Circlet’s circlet/middleware function to coerce an argument into a piece of middleware. Middleware pieces are often higher-order functions (meaning that they return another function). This allows for parameterization at creation time.

Provided middleware

There are three basic pieces of middleware provided by Circlet:

  • (circlet/router routes) simple routing facility. This function takes a Janet table containing the routes. Each key should be a Janet string matching a URI (e.g. ”/“, ”/posts") with a value that is a function of the same form as the handler function described above.
  • (circlet/logger nextmw) simple logging facility. This function prints the request info on stdout. The only argument is the next middleware.
  • (circlet/cookies nextmw) middleware which extracts the cookies from the HTTP header and stores the value under the :cookies key in the request object.


The below example starts a very simple web server on port 8000.

(import circlet)

(defn myserver
 "A simple HTTP server" [request]
 {:status 200
  :headers {"Content-Type" "text/html"} :body "<!doctype html><html><body><h1>Hello.</h1></body></html>"})

(circlet/server myserver 8000)



Building requires Janet to be installed on the system, as well as the jpm tool (installed by default with Janet).

jpm build

You can also just run jpm to see a list of possible build commands.


Run a server on localhost with the following command

jpm test

This example is more involved, and shows all the functionality described in this document.


Unlike janet, Circlet is licensed under the GPL license in accordance with mongoose.


HTTP server library for janet




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