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Carnap is a free and open-source Haskell framework for creating and exploring formal languages, logics, and semantics. It lets you quickly and straightforwardly define languages, construct logics for those languages, and stipulate their semantics. Carnap then uses your specifications to figure out how to check proofs in a variety of formal systems using your logic, how to find the meanings of compound expressions, and a whole lot more.

Carnap's primary application at the moment is powering, a website supporting online logic instruction and learning. If you'd like to seem some demos and more general information, head over to If you'd like to learn more about using the server as an instructor, head over to the documentation collection at

If you're interested in contributing to software development or modifying the software, read on. This README will help you set up a development environment for building the server-side and client-side components used on

LTI Setup (for Learning Management System administrators)

Carnap supports LTI 1.3 for login, allowing it to be placed directly in a Canvas course. No previous versions superseded by LTI 1.3 including LTI 1.0, 1.1, or 2.0 are supported.

Various major LMS implementations including Moodle and Canvas support LTI 1.3 natively.'s configuration parameters are:

A JSON file for easy configuration via Canvas is provided at ./carnap-lti-canvas.json.

After this configuration is set up, it must be configured on the Carnap instance administration page at /master_admin. For setup with the public Carnap instance at, contact Graham with the following details from your LMS:

  • Public JWKs URL
  • Authorization Redirect URL
  • Client ID

For more details on Canvas setup with LTI 1.3, see:


General information

The current development environment is based on Nix. For general background on Nix, take a look at or

Building on Windows natively is not supported, however, Carnap works well in the Windows Subsystem for Linux, and has been tested and confirmed working in WSL 2. The steps for Linux work as-written in WSL 2.

Nix is used to speed up builds and avoid having to compile dependencies, instead using cached built versions from nixpkgs. It also makes it easy to Dockerize (see also NixOps) and implement CI.

Carnap has a GHCJS (client side) and a GHC (server side) part.

Building Carnap-Server depends on the client side components being available at client-out or otherwise linked into its static folder. This will be handled automatically by the approaches to building the client listed below, but the client needs to be built before the server.

Nix does not support incremental package builds, so the suggested development workflow is to use cabal in a shell. A Makefile has been provided to make this easier by shortening the commands needed to be typed.

In this documentation, lines starting with $ mean that the part after the $ is run in a shell natively on your system. Lines starting with [nix-shell]$ are intended to be run in a nix-shell shell, which will have that prompt.

Nix setup

Install Nix with your non-root user account:

$ bash <(curl -L

macOS 10.15 (Catalina) users may need to follow additional steps, documented at

You can significantly speed up builds by using binaries for Carnap dependencies from Carnap's Cachix instance, which has support for both Linux and macOS. To use it:

$ # Install Cachix:
$ nix-env -iA cachix -f
$ # Use the Carnap Cachix
$ cachix use carnap

Then build as normal.

A quick aside (for those who are curious about what these commands are doing): Nix, by default, reads shell.nix if you call it through nix-shell, and default.nix if you call it with nix-build or others. That's why there are attribute names being used with -A for the nix-build commands, but not for nix-shell.

Build the front end

This builds the Carnap-GHCJS package under ghcjs along with its dependencies. (note: takes about 9-10GB of RAM at the final linking stage). This is often faster than building it with Cabal if you only intend to work on Carnap-Server, since the CI build server produces cached built versions of the client side components, allowing the build step to be skipped.

$ nix-build -j4 -A client -o client-out

Build Carnap-Server for deployment

This will also automatically build the client and any dependencies if necessary.

$ nix-build -j4 -A server -o server-out

Get a shell with GHC, for server development

This creates a shell in an environment providing the required dependencies for Carnap-Server, along with development tools such as cabal, ghc and others appropriate for server side development. Nix shells use versions of Haskell components and tools isolated from those available on the host system.

This requires a client be built, either using Nix or using Cabal (see below).

Using make aliases:

$ make shell-ghc
[nix-shell:Carnap]$ make build-ghc
[nix-shell:Carnap]$ make run

You should then be able to access your development server at http://localhost:3000

To perform the process manually, first make sure to copy Carnap-Server/config/settings-example.yml to Carnap-Server/config/settings.yml, and to create a data directory (in the below, called dataroot). Then run:

$ nix-shell
[nix-shell:Carnap]$ cabal new-build -f dev all
[nix-shell:Carnap]$ cd Carnap-Server
[nix-shell:Carnap-Server]$ APPROOT="http://localhost:3000" DATAROOT="../dataroot" BOOKROOT="../Carnap-Book/" cabal run -f dev Carnap-Server

Get a shell with GHCJS, for client side development

As in the section above, this will make a shell with Haskell development tools and the required dependencies to work on Carnap components. This one provides the GHCJS tooling and dependencies for the client side.

Using make aliases:

$ make shell-ghcjs
[nix-shell:Carnap]$ make build-ghcjs


$ nix-shell --arg ghcjs true
$ cabal --project-file=cabal-ghcjs.project --builddir=dist-ghcjs new-build all

Haskell Language Server

The Nix infrastructure for Carnap provides the Haskell Language Server by default for development on Carnap-Server, allowing for completion, type information and more. It is not yet working on the exclusively-GHCJS-built components.

To use it:

Make the following shell script somewhere in $PATH, called hls-nix:


nix-shell --run "haskell-language-server-wrapper $@"

Then, configure per the information in the README. For example, for neovim with Coc, the following configuration should be put in the file brought up by :CocConfig:

"languageserver": {
  "haskell": {
    "command": "hls-nix",
    "args": ["--lsp"],
    "rootPatterns": ["*.cabal", "stack.yaml", "cabal.project", "package.yaml", "hie.yaml"],
    "filetypes": ["haskell", "lhaskell"]

NOTE: we are using the hls-nix wrapper script as the command here, so HLS picks up the dependencies for Carnap-Server from Nix since it is run in a nix-shell.

NOTE: Carnap-Server/config/settings.yml needs to be present for Carnap-Server to work in HLS (just copy it from the example settings file).



There is experimental Docker support for Carnap. Currently, diagrams support is broken, which impacts some chapters of the Carnap textbook.

Images are available via the GitHub container registry at

Note that Carnap docker images are about 3GB uncompressed and about 500MB to download.

If you'd like to build an image locally for development/testing, run:

nix-build release.nix -A docker -o docker-out

then load it into the docker daemon with:

docker image load -i docker-out

It will be available under the image name carnap:latest.

To run Carnap under docker:

docker run --rm -v carnap_data:/data -e APPROOT=

For production deployment, set the environment variable SQLITE=false and supply PGUSER, PGPASS, PGHOST, and if required, PGPORT and PGDATABASE for your postgresql database instance.

The docker building setup does not require KVM support in the host kernel, and if you wish to build your images on a machine without it, run nix-build with the added argument --arg hasKvm false.

Maintainer information

CI requirements

CI requires that a Cachix signing key for the Cachix cache be supplied in the CACHIX_SIGNING_KEY Secret in the GitHub repository settings for artifacts to be pushed to Cachix.

Generate the .nix files in a subproject

For example:

$ cd Carnap-Client; cabal2nix . > Carnap-Client.nix

Updating nixpkgs

The version of nixpkgs used is pinned to a fixed git commit to ensure that no unexpected issues happen with our builds. Occasionally it should be updated, then Carnap build reattempted. Currently we use a system of two versions of nixpkgs, nixpkgs and nixpkgs-stable, with nixpkgs used for Carnap-Server, having the latest packages including haskell-language-server, and nixpkgs-stable providing a working ghcjs (since it is supposedly broken in the latest nixpkgs). nixpkgs-stable as of the time of this writing is pinned to nixos-20.03 whereas nixpkgs is on nixpkgs-unstable, a tested-but-rolling-release build.

We now use niv to manage our nixpkgs versions. It is available from either development shell.

Run niv update nixpkgs or niv update nixpkgs-stable to update the respective pinned nixpkgs versions to the latest in their branch.

Files in nix/

If you need to get a package from Hackage, use cabal2nix:

$ cabal2nix cabal://ghcjs-dom- | tee ghcjs-dom.nix
$ # or for ghcjs
$ cabal2nix --compiler ghcjs cabal://ghcjs-dom- | tee ghcjs-dom-ghcjs.nix

There is also work-on-multi.nix which provides shells for working on multi-piece Haskell projects. It's from reflex-platform, but freed from its dependencies on that project and simplified slightly.


Every instance of doJailbreak is disabling version checking in Cabal. Most of the time this works fine, but it indicates bugs in the packages' Cabal version restrictions.

Carnap currently uses an older ghcjs-dom from 2016, which is largely fine on web but requires webkitgtk24x-gtk3, which was been removed from Nix in late 2019. It is thus currently not possible to build the Carnap front end natively (i.e. for embedding in non-browser applications) pending this update.

A bunch of packages have broken unit tests on ghcjs. All tests on ghcjs are accordingly disabled (also, for performance reasons).

Suggested Nix Resources

Blog post on Carnap's nix implementation:

Tutorial some of the Nix here is loosely based on:

reflex-platform documentation that inspired our shell setup:

User's guide to the Haskell infrastructure:

Source code to the Haskell infrastructure (has good doc comments):

Basic information about the builder and e.g. how to build manually:


A formal logic framework that runs in the browser




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