Announcing: Yesod 1.1, Persistent 1.0, and a new initiative

August 7, 2012

GravatarBy Michael Snoyman

The Yesod team is happy to announce the 1.1 release of Yesod, and the 1.0 release of Persistent. Yesod is a web application framework for Haskell, which allows you to leverage type safety to help mitigate entire classes of bugs, while allowing you to write your code quickly and concisely. Persistent is Yesod's most commonly used data storage layer, featuring full compile time checking of database interactions.

These releases maintain a large degree of backwards compatibility, and as such, upgrade should be fairly simple. As users begin the upgrade process, we'll put together a collection of notes for handling upgrades and post them here.



  • Upgrade to conduit 0.5
  • Hierarchical routing.
  • Control of sitewide Hamlet settings in the scaffolding.
  • Easily add additional template languages in Settings.hs.
  • yesod add-handler automates the process of adding a new route, creating the handler module, creating stub handler functions, and updating Application.hs and the cabal file.
  • yesod keter builds a keter bundle. With a config setting, it will also upload it for you.
  • By default, response bodies are now fully evaluated before sending to avoid empty responses when pure code throws an exception. DontFullyEvaluate is provided to override this default.
  • Better control of uploaded file handling, defaulting to temporary file system storage.


  • Upgrade to conduit 0.5
  • Sum types
  • Support for sqltype=... attribute.

A new initiative: add-ons

Yesod has a very strong set of core packages. We provide many features out of the box with a standard platform install. And our development approach is very open, allowing many people to submit features back into these core packages.

Overall, this has been a great approach. Users have a well known set of libraries they can rely on, and those libraries are very featureful. However, this centralized development approach has a few downsides:

  • It doesn't give much room for experimentation. Either the code makes it into the core packages, or it's not in at all.

  • There are only so many hours in the day, and only so many people on the Yesod core team. If every feature anyone ever dreams of has to go through us, we'll become a bottleneck.

We're not actually dealing with any form of technical problem. Yesod is already designed to be highly modular. Most of the "core components" of Yesod, like forms and authentication, are provided as separate packages anyway. There's actually very little in the yesod-core package itself.

Greg and I have discussed this some, and we think it's time to start a new initiative in the community: writing add-ons. An add-on approach has a number of advantages:

  • A new contributor doesn't have to deal with any of the complexity of the main Yesod libraries, instead getting to focus on much smaller pieces of code.

  • There's no overview process necessary: you can write your code, put it on Github/BitBucket/wherever, upload to Hackage, and email the mailing list, without anyone needing to review your code. (You can of course still ask us to review code if you want.)

  • If you have some crazy idea you'd like to experiment with, you can do so. Since it's not part of the core packages, you can play around as much as you want.

So how do you create an add-on to Yesod? I'd say it breaks down into one of three categories:

A collection of helper functions

A great example of this is yesod-goodies. The idea is to just take some commonly used code, or some code to interface with an external system like Gravatar, and package it up. Often times, this code won't even have anything to do with Yesod specifically: the Gravatar code in yesod-goodies, for example, is just interested in generating the appropriate URLs.

There are some things to keep in mind when writing this kind of code, such as keeping your type variables more generic. For example, in a typical Yesod site, you could write something like:

someFunction :: Int -> Handler RepHtml

But in generic code, you can't use the Handler type alias. Instead, you'll need to use GHandler (generic handler), and leave type variables empty for the sub and master sites, e.g.:

someFunction :: Int -> GHandler sub master RepHtml

Pre-built Widgets

Remember that Yesod was built with reusable components in mind. One of the strongest examples is Widgets, allowing you to bundle together HTML, CSS, and Javascript. One possibility is to take some external API (like Google Maps) and package up some front end to it (for example, Haskellers has some location selecting code in it).

Like Handler, you'll need to change your type signature a bit, e.g.:

mapSelector :: GWidget sub master ()

And you can also use type classes to specify some additional requirements. For example, if you're going to be using jQuery, you could write something like:

usesJquery :: YesodJquery master => GWidget sub master ()
usesJquery = do
    master <- lift getYesod
    addScriptEither $ urlJqueryJs master


The other major reusable component in Yesod is subsites. This allows you to package up entire sets of route handlers and reuse them across multiple sites. There's a very solid example of a chat subsite in the Yesod book. Other existing subsites are for authentication and static files. And another subsite others have been working on is an admin subsite (similar to Django's admin system).


I think this initiative will allow more users to become involved in Yesod development, and let more interesting bits of code come out of the woodwork. None of this is to say that the Yesod core team is going to start turning away pull requests. Quite the contrary, we'll still happily be accepting added features that are solid and stable. (And of course bug fixes and performance enhancements will continue as well.) This also doesn't mean you're on your own: if you have an idea and want some help bringing it to fruition, bring it up on the mailing list, there are lots of great people there who can give amazing feedback.

As this add-on process progresses, I'm also hoping users start writing more blog posts about their experiences in writing these add-ons, and eventually we can get a community-driven set of guides for new add-on writers. I also believe that in the near future, there will be even more advantages available to add-on authors, but that idea will have to wait for a later blog post.


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