Scaffolding and the Site Template

So you’re tired of running small examples, and ready to write a real site? Then you’re at the right chapter. Even with the entire Yesod library at your fingertips, there are still a lot of steps you need to go through to get a production-quality site setup:

  • Config file parsing

  • Signal handling (*nix)

  • More efficient static file serving

  • A good file layout

The scaffolded site is a combination of many Yesoders' best practices combined together into a ready-to-use skeleton for your sites. It is highly recommended for all sites. This chapter will explain the overall structure of the scaffolding, how to use it, and some of its less-than-obvious features.

For the most part, this chapter will not contain code samples. It is recommended that you follow along with an actual scaffolded site.

How to Scaffold

The yesod-bin package installs an executable (conveniently named yesod as well). This executable provides a few commands (run stack exec -- yesod --help to get a list). In order to generate a scaffolding, the command is stack new my-project yesodweb/postgres && cd my-project. This will generate a scaffolding site with a postgres database backend in a directory named my-project. You can see the other available templates using the command stack templates.

The key thing differing in various available templates (from the stack templates command) is the database backend. You get a few choices here, including SQL backends, as well as "yesodweb/simple" template to include no database support. This last option also turns off a few extra dependencies, giving you a leaner overall site. The remainder of this chapter will focus on the scaffoldings for one of the database backends. There will be minor differences for the yesodweb/simple backend.

After creating your files, install the yesod command line tool inside the project: stack install yesod-bin --install-ghc. Then do a stack build inside the directory. In particular, the commands provided will ensure that any missing dependencies are built and installed.

Finally, to launch your development site, you would use stack exec -- yesod devel. This site will automatically rebuild and reload whenever you change your code.

File Structure

The scaffolded site is built as a fully cabalized Haskell package. In addition to source files, config files, templates, and static files are produced as well.

Cabal file

Whether directly using stack, or indirectly using stack exec -- yesod devel, building your code will always go through the cabal file. If you open the file, you’ll see that there are both library and executable blocks. If the library-only flag is turned on, then the executable block is not built. This is how yesod devel calls your app. Otherwise, the executable is built.

The library-only flag should only be used by yesod devel; you should never be explicitly passing it into cabal. There is an additional flag, dev, that allows cabal to build an executable, but turns on some of the same features as the library-only flag, i.e., no optimizations and reload versions of the Shakespearean template functions.

In general, you will build as follows:

  • When developing, use yesod devel exclusively.

  • When building a production build, perform stack clean && stack build. This will produce an optimized executable in your dist folder. (You can also use the yesod keter command for this.)

You might be surprised to see the NoImplicitPrelude extension. We turn this on since the site includes its own module, Import, with a few changes to the Prelude that make working with Yesod a little more convenient.

The last thing to note is the exposed-modules list. If you add any modules to your application, you must update this list to get yesod devel to work correctly. Unfortunately, neither Cabal nor GHC will give you a warning if you forgot to make this update, and instead you’ll get a very scary-looking error message from yesod devel.

Routes and entities

Multiple times in this book, you’ve seen a comment like “We’re declaring our routes/entities with quasiquotes for convenience. In a production site, you should use an external file.” The scaffolding uses such an external file.

Routes are defined in config/routes, and entities in config/models. They have the exact same syntax as the quasiquoting you’ve seen throughout the book, and yesod devel knows to automatically recompile the appropriate modules when these files change.

The models files is referenced by Model.hs. You are free to declare whatever you like in this file, but here are some guidelines:

  • Any data types used in entities must be imported/declared in Model.hs, above the persistFileWith call.

  • Helper utilities should either be declared in Import.hs or, if very model-centric, in a file within the Model folder and imported into Import.hs.

Foundation and Application modules

The mkYesod function which we have used throughout the book declares a few things:

  • Route type

  • Route render function

  • Dispatch function

The dispatch function refers to all of the handler functions, and therefore all of those must either be defined in the same file as the dispatch function, or be imported into the module containing the dispatch function.

Meanwhile, the handler functions will almost certainly refer to the route type. Therefore, they must be either in the same file where the route type is defined, or must import that file. If you follow the logic here, your entire application must essentially live in a single file!

Clearly this isn’t what we want. So instead of using mkYesod, the scaffolding site uses a decomposed version of the function. Foundation.hs calls mkYesodData, which declares the route type and render function. Since it does not declare the dispatch function, the handler functions need not be in scope. Import.hs imports Foundation.hs, and all the handler modules import Import.hs.

In Application.hs, we call mkYesodDispatch, which creates our dispatch function. For this to work, all handler functions must be in scope, so be sure to add an import statement for any new handler modules you create.

Other than that, Application.hs is pretty simple. It provides two primary functions: getApplicationDev is used by yesod devel to launch your app, and makeApplication is used by the executable to launch.

Foundation.hs is much more exciting. It:

  • Declares your foundation datatype

  • Declares a number of instances, such as Yesod, YesodAuth, and YesodPersist

  • Imports the messages files. If you look for the line starting with mkMessage, you will see that it specifies the folder containing the messages (messages/) and the default language (en, for English).

This is the right file for adding extra instances for your foundation, such as YesodAuthEmail or YesodBreadcrumbs.

We’ll be referring back to this file later, as we discussed some of the special implementations of Yesod typeclass methods.


The Import module was born out of a few commonly recurring patterns.

  • I want to define some helper functions (maybe the <> = mappend operator) to be used by all handlers.

  • I’m always adding the same five import statements (Data.Text, Control.Applicative, etc) to every handler module.

  • I want to make sure I never use some evil function (head, readFile, …) from Prelude.

The solution is to turn on the NoImplicitPrelude language extension, re-export the parts of Prelude we want, add in all the other stuff we want, define our own functions as well, and then import this file in all handlers.

Handler modules

Handler modules should go inside the Handler folder. The site template includes one module: Handler/Home.hs. How you split up your handler functions into individual modules is your decision, but a good rule of thumb is:

  • Different methods for the same route should go in the same file, e.g. getBlogR and postBlogR.

  • Related routes can also usually go in the same file, e.g., getPeopleR and getPersonR.

Of course, it’s entirely up to you. When you add a new handler file, make sure you do the following:

  • Add it to version control (you are using version control, right?).

  • Add it to the cabal file.

  • Add it to the Application.hs file.

  • Put a module statement at the top, and an import Import line below it.

You can use the stack exec -- yesod add-handler command to automate the last three steps.


It’s very common to want to include CSS and Javascript specific to a page. You don’t want to have to remember to include those Lucius and Julius files manually every time you refer to a Hamlet file. For this, the site template provides the widgetFile function.

If you have a handler function:

getHomeR = defaultLayout $(widgetFile "homepage")

, Yesod will look for the following files:

  • templates/homepage.hamlet

  • templates/homepage.lucius

  • templates/homepage.cassius

  • templates/homepage.julius

If any of those files are present, they will be automatically included in the output.


One of the first things you’re going to want to customize is the look of your site. The layout is actually broken up into two files:

  • templates/default-layout-wrapper.hamlet contains just the basic shell of a page. This file is interpreted as plain Hamlet, not as a Widget, and therefore cannot refer to other widgets, embed i18n strings, or add extra CSS/JS.

  • templates/default-layout.hamlet is where you would put the bulk of your page. You must remember to include the widget value in the page, as that contains the per-page contents. This file is interpreted as a Widget.

Also, since default-layout is included via the widgetFile function, any Lucius, Cassius, or Julius files named default-layout.* will automatically be included as well.

Static files

The scaffolded site automatically includes the static file subsite, optimized for serving files that will not change over the lifetime of the current build. What this means is that:

  • When your static file identifiers are generated (e.g., static/mylogo.png becomes mylogo_png), a query-string parameter is added to it with a hash of the contents of the file. All of this happens at compile time.

  • When yesod-static serves your static files, it sets expiration headers far in the future, and includes an etag based on a hash of your content.

  • Whenever you embed a link to mylogo_png, the rendering includes the query-string parameter. If you change the logo, recompile, and launch your new app, the query string will have changed, causing users to ignore the cached copy and download a new version.

Additionally, you can set a specific static root in your Settings.hs file to serve from a different domain name. This has the advantage of not requiring transmission of cookies for static file requests, and also lets you offload static file hosting to a CDN or a service like Amazon S3. See the comments in the file for more details.

Another optimization is that CSS and Javascript included in your widgets will not be included inside your HTML. Instead, their contents will be written to an external file, and a link given. This file will be named based on a hash of the contents as well, meaning:

  1. Caching works properly.

  2. Yesod can avoid an expensive disk write of the CSS/Javascript file contents if a file with the same hash already exists.

Finally, all of your Javascript is automatically minified via hjsmin.

Environment variables

Scaffolded apps are preconfigured to support these environment variables :

  • YESOD_STATIC_DIR : Path of static directory.

  • YESOD_HOST : Host/interface the server should bind to.

  • YESOD_PORT : Port to listen on.

  • YESOD_IP_FROM_HEADER : Get the IP address from the header when logging. Useful when sitting behind a reverse proxy.

They are listed in file /config/settings.yml, where their default values can be defined. Other variables can be added by uncommenting dedicated lines :

  • YESOD_APPROOT : Explicit base for all generated URLs.

  • PORT : Port for Keter.


The purpose of this chapter was not to explain every line that exists in the scaffolded site, but instead to give a general overview to how it works. The best way to become more familiar with it is to jump right in and start writing a Yesod site with it.