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Web Server Template

This is a template implementation of a Yesod Web Server.

  • Haskell code for a Yesod Web Server with all best practices
  • Integration tests for each handler
  • OptParse for the Web server
  • A nix build
  • An example cast for documentation
  • CI
    • Stack-based CI
    • Nix-based CI
  • Pre-commit hooks


This template is not free to use. See for more information.

Copyright (c) 2020-2022 Tom Sydney Kerckhove.

All Rights Reserved.


To use this template in a new project, choose the name for your project, for example homeless-shelter. Then use template-filler to use the template, like this:

template-filler --source /path/to/this/template-web-server --destination /path/to/your/homeless-shelter --find FooBar --replace HomelessShelter

Template overview

There is a single Haskell package in foo-bar-web-server. It contains the following structure:

  • The entry point in Foo.Bar.Web.Server
  • The Yesod App type in Foo.Bar.Web.Server.Foundation
  • The Handlers in Foo.Bar.Web.Server.Handler.<HandlerName>
  • OptParse in Foo.Bar.Web.Server.OptParse
  • Declarations of static assets in Foo.Bar.Web.Server.Static.TH
  • The development constant in Foo.Bar.Web.Server.Constants
  • Test utilities in Foo.Bar.Web.Server.TestUtils
  • Tests per handler Foo.Bar.Web.Server.Handler.<HandlerName>Spec


The option parsing is based on the option parsing template. It is included in this template so you will not need to also buy the option parsing template.

For more information about how to use the option parsing, follow the instructions in template-web-server/src/Foo/Bar/Web/Server/OptParse.hs.

Nix build

If you don't need a nix build, remove these files:

rm -rf *.nix nix .github/workflows/nix.yaml

In nix/nixpkgs-version.nix, we pin a nixpkgs commit. In nix/pkgs.nix we define our own 'version' of the nixpkgs by adding our own overlays. The project overlay is defined in nix/overlay.nix.

See the instructions in nix/overlay.nix for more details.


CI is set up for both a stack build and a nix build. See .github/workflows for more details.

The stack build should "just work".

For the nix build to work, there is a manual step that you need to go through: First, make a cachix cache at Put its name in the right places within .github/workflows/nix.yaml. Then put its signing key in the 'Secrets' part of your repository on github.

Workflow examples

Adding a Handler

  1. Add the route in foo-bar-web-server/routes.txt.

  2. Add the handler in a new module Foo.Bar.Web.Server.Handler.<HandlerName> with a function according to the route definition:

    get<HandlerName> :: Handler Html
  3. Add tests in Foo.Bar.Web.Server.Handler.<HandlerName>Spec.