A tasty Haskell front-end framework


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LTS Haskell 22.25:
Stackage Nightly 2024-06-14:
Latest on Hackage:

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BSD-3-Clause licensed and maintained by David M. Johnson
This version can be pinned in stack with:miso-,7493

Miso is a small “isomorphicHaskell front-end framework for quickly building highly interactive single-page web applications. It features a virtual-dom, diffing / patching algorithm, attribute and property normalization, event delegation, event batching, SVG, Server-sent events, Websockets, type-safe servant-style routing and an extensible Subscription-based subsystem. Inspired by Elm, Redux and Bobril. Miso is pure by default, but side effects (like XHR) can be introduced into the system via the Effect data type. Miso makes heavy use of the GHCJS FFI and therefore has minimal dependencies. Miso can be considered a shallow embedded domain-specific language for modern web programming.

Table of Contents

Quick start

To get started quickly building applications, we recommend using the stack or nix package managers. Obtaining GHCJS is required as a prerequisite. stack and nix make this process easy, if you’re using cabal we assume you have obtained GHCJS by other means.

All source code depicted below for the quick start app is available here.


We recommend using nix when working with miso, but it is just as fine to use stack. To build the sample-app with nix, execute the command below:

git clone https://github.com/dmjio/miso && cd miso/sample-app && nix-build

To develop with nix run the below command (this will put you into a shell where you can iteratively develop the project with cabal build).

git clone https://github.com/dmjio/miso && cd miso/sample-app && nix-shell -A env

For more information on using nix w/ miso, see the nix section below

To build the sample-app with stack, execute the command below:

git clone https://github.com/dmjio/miso && cd miso/sample-app && stack setup && stack build

Note: It’s important to ensure that you don’t have a global cabal-install present on your system. This could cause build problems. If you see an error like this (below), try deleting your global cabal-install.

exit status: 1
stderr: solver must be one of: modular
CallStack (from HasCallStack):
  error, called at libraries/Cabal/Cabal/Distribution/ReadE.hs:46:24 in Cabal-

For more information on using stack w/ miso, see the stack section below


In the miso repository there is a folder named stack with “known to work” configurations for GHCJS. One stack file exists for both the 7.10.3 and 8.0.1 versions of GHCJS. In general, we recommend developing with the 7.10.3 version since it currently supports GHCJSi (a REPL that connects to the browser by way of a nodejs web server using socket.io) and building with the 8.0.1 version (if possible). For more information on using stack with GHCJS, please consult the GHCJS section of the stack docs.

To begin, create the following directory layout

➜  mkdir app && touch app/{Main.hs,app.cabal,stack.yaml} && tree app
|-- Main.hs
|-- app.cabal
`-- stack.yaml

Add a stack.yaml file that uses a recent version of miso.

➜  cat app/stack.yaml
resolver: lts-6.30
compiler: ghcjs-
compiler-check: match-exact

 - '.'
 - miso-

		 url: http://ghcjs.tolysz.org/lts-6.30-9006030.tar.gz
		 sha1: 2371e2ffe9e8781808b7a04313e6a0065b64ee51

Add a cabal file

➜  cat app/*.cabal
name:                app
synopsis:            First miso app
category:            Web
build-type:          Simple
cabal-version:       >=1.10

executable app
  main-is:             Main.hs
  build-depends:       base, miso
  default-language:    Haskell2010

Add the source from Sample Application to app/Main.hs

Run stack setup. This might take a long time, since it will have to build GHCJS.

stack setup

Run stack build to get the static assets

stack build

See the result

open $(stack path --local-install-root)/bin/app.jsexe/index.html

Using GHCJSi

stack ghci

If that warns with socket.io not found, browser session not available, you’ll need to install socket.io

npm install socket.io

and update your NODE_PATH

export NODE_PATH=$(pwd)/node_modules

Now you should be connected, and the app viewable in GHCJSi (open http://localhost:6400).

➜  stack ghci
app- initial-build-steps (exe)
Configuring GHCi with the following packages: app
GHCJSi, version http://www.github.com/ghcjs/ghcjs/  :? for help
[1 of 1] Compiling Main             ( /Users/david/Desktop/miso/sample-app/Main.hs, interpreted )
socket.io found, browser session available at http://localhost:6400
Ok, modules loaded: Main.
*Main> main
browser connected, code runs in browser from now on


Nix is a more powerful option for building web applications with miso since it encompasses development workflow, configuration management, and deployment. The source code for haskell-miso.org is an example of this.

If unfamiliar with nix, we recommend @Gabriel439’s “Nix and Haskell in production” guide.

To get started, we will use the cabal2nix tool to convert our Cabal file into a nix derivation (named app.nix). We’ll then write a file named default.nix, which is used for building our project (via nix-build) and development (via nix-shell).

To begin, make the following directory layout:

➜  mkdir app && touch app/{Main.hs,app.cabal,default.nix,app.nix} && tree app
|-- Main.hs
|-- app.cabal
|-- default.nix
`-- app.nix

Add a cabal file

➜  cat app/*.cabal
name:                app
synopsis:            First miso app
category:            Web
build-type:          Simple
cabal-version:       >=1.10

executable app
  main-is:             Main.hs
  build-depends:       base, miso
  default-language:    Haskell2010

Use cabal2nix to generate a file named app.nix that looks like below.

➜  cabal2nix . --compiler ghcjs > app.nix
➜  cat app.nix
{ mkDerivation, base, miso, stdenv }:
mkDerivation {
  pname = "app";
  version = "";
  src = ./.;
  isLibrary = false;
  isExecutable = true;
  executableHaskellDepends = [ base miso ];
  description = "First miso app";
  license = stdenv.lib.licenses.bsd3;

Write a default.nix (which calls app.nix), this fetches a recent version of miso.

{ pkgs ? import ((import <nixpkgs> {}).fetchFromGitHub {
	owner = "NixOS";
	repo = "nixpkgs";
	rev = "a0aeb23";
	sha256 = "04dgg0f2839c1kvlhc45hcksmjzr8a22q1bgfnrx71935ilxl33d";
  result = import (pkgs.fetchFromGitHub {
	owner = "dmjio";
	repo = "miso";
	sha256 = "1l1gwzzqlvvcmg70jjrwc5ijv1vb6y5ljqkh7rxxq7hkyxpjyx9q";
	rev = "95f6bc9b1ae6230b110358a82b6a573806f272c2";
  }) {};
in pkgs.haskell.packages.ghcjs.callPackage ./app.nix {
  miso = result.miso-ghcjs;

Add the source from Sample Application to app/Main.hs

Build the project


Open the result

open ./result/bin/app.jsexe/index.html

For development with nix, it’s important to have cabal present for building. This command will make it available in your PATH.

nix-env -iA cabal-install -f '<nixpkgs>'

To be put into a shell w/ GHCJS and all the dependencies for this project present, use nix-shell.

nix-shell -A env

To open GHCJSi (NODE_PATH should already be set properly)

$ cabal configure --ghcjs
$ cabal repl
Package has never been configured. Configuring with default flags. If this
fails, please run configure manually.
Resolving dependencies...
Configuring app-
Preprocessing executable 'app' for app-
GHCJSi, version 0.2.0-7.10.3: http://www.github.com/ghcjs/ghcjs/  :? for help
[1 of 1] Compiling Main             ( Main.hs, interpreted )
Ok, modules loaded: Main.
browser connected, code runs in browser from now on


The latest stable version of miso will be available on Hackage. To build with cabal, we assume ghcjs is in your PATH and ghcjs-base is present in your ghcjs-pkg list.

cabal sandbox init
cabal install --ghcjs
cabal build
open dist/build/app/app.jsexe/index.html

GHCJSi Caveats

If you run main in GHCJSi, interrupt it and then run it again, you will end up with two copies of your app displayed above each other. As a workaround, you can use clearBody >> main which will completely clear the document body before rendering your application.


For constructing client and server applications, we recommend using one cabal file with two executable sections, where the buildable attribute set is contingent on the compiler. An example of this layout is here. For more info on how to use stack with a client/server setup, see this link. For more information on how to use nix with a client/server setup, see the nix scripts for https://haskell-miso.org.












Canvas 2D



File Reader





Sample application

-- | Haskell language pragma
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE RecordWildCards #-}

-- | Haskell module declaration
module Main where

-- | Miso framework import
import Miso
import Miso.String

-- | Type synonym for an application model
type Model = Int

-- | Sum type for application events
data Action
  = AddOne
  | SubtractOne
  | NoOp
  | SayHelloWorld
  deriving (Show, Eq)

-- | Entry point for a miso application
main :: IO ()
main = startApp App {..}
    initialAction = SayHelloWorld -- initial action to be executed on application load
    model  = 0                    -- initial model
    update = updateModel          -- update function
    view   = viewModel            -- view function
    events = defaultEvents        -- default delegated events
    subs   = []                   -- empty subscription list
    mountPoint = Nothing          -- mount point for application (Nothing defaults to 'body')

-- | Updates model, optionally introduces side effects
updateModel :: Action -> Model -> Effect Action Model
updateModel AddOne m = noEff (m + 1)
updateModel SubtractOne m = noEff (m - 1)
updateModel NoOp m = noEff m
updateModel SayHelloWorld m = m <# do
  putStrLn "Hello World" >> pure NoOp

-- | Constructs a virtual DOM from a model
viewModel :: Model -> View Action
viewModel x = div_ [] [
   button_ [ onClick AddOne ] [ text "+" ]
 , text (ms x)
 , button_ [ onClick SubtractOne ] [ text "-" ]

Building examples

The easiest way to build the examples is with the nix package manager

git clone https://github.com/dmjio/miso && cd miso && nix-build

This will build all examples and documentation into a folder named result

➜  miso git:(master) ✗ tree result -d
|-- doc
|   |-- x86_64-osx-ghc-8.0.2
|   |   `-- miso-
|   |       `-- html
|   |           `-- src
|   `-- x86_64-osx-ghcjs-0.2.0-ghc7_10_3
|       `-- miso-
|           `-- html
|               `-- src
|-- examples
|   |-- mario.jsexe
|   |   `-- imgs
|   |       |-- jump
|   |       |-- stand
|   |       `-- walk
|   |-- router.jsexe
|   |-- simple.jsexe
|   |-- tests.jsexe
|   |-- todo-mvc.jsexe
|   `-- websocket.jsexe

To see examples, we recommend hosting them with a webserver

cd result/examples/todo-mvc.jsexe && python -m SimpleHTTPServer
Serving HTTP on port 8000 ...


The core algorithmic component of miso is diff.js. It is responsible for all DOM manipulation that occurs in a miso application and has 100% code coverage. Tests and coverage made possible using jsdom and jest.

To run the tests and build the coverage report:

cd miso/tests
npm i
npm run test
## Or by using `yarn` instead of `npm`:
# yarn
# yarn test


Isomorphic javascript is a technique for increased SEO, code-sharing and perceived page load times. It works in two parts. First, the server sends a pre-rendered HTML body to the client’s browser. Second, after the client javascript application loads, the pointers of the pre-rendered DOM are copied into the virtual DOM, and the application proceeds as normal. All subsequent page navigation is handled locally by the client, avoiding full-page postbacks as necessary.

The miso function is used to perform the pointer-copying behavior client-side.

For more information on how miso handles isomorphic javascript, we recommend this tutorial.

Pinning nixpkgs

By default miso uses a known-to-work, pinned version of nixpkgs. To override this to your system’s version of nixpkgs write:

nix-build --arg nixpkgs 'import <nixpkgs> {}'

Binary cache

nix users on a Linux distro can take advantage of a binary cache for faster builds. To use the binary cache simply append https://hydra.dmj.io/nix-cache-info during all nix-shell and/or nix-build invocations.

nix-build --option extra-binary-caches https://hydra.dmj.io

Alternatively, add https://hydra.dmj.io to your list of local binary caches in nix.conf (usually found in /etc/nix/nix.conf), and it will automatically be used on all invocations of nix-build and/or nix-shell.

binary-caches = https://hydra.dmj.io/ https://cache.nixos.org/


According to benchmarks, miso is among the fastest functional programming web frameworks, second only to Elm.




Feel free to dive in! Open an issue or submit PRs.

See CONTRIBUTING for more info.


BSD3 © David Johnson