The Snap team is delighted to announce the anxiously awaited release of version 1.0 of the Snap Web Framework for Haskell. Snap has been used in stable production applications for years now, and with this release we’re updating our version number to reflect the stability and commitment to backwards compatibility that our users depend on. Here is a summary of the major changes:

The Details

Now backed by io-streams

Snap’s web server has been overhauled, replacing the enumerator package with the newer, leaner, faster, and easier to use io-streams. If you were using of Snap’s low-level enumerator functions, those will need to be migrated to io-streams. Otherwise there should be few interface changes.

More modular project template infrastructure

The snap executable that generates project templates has been moved from the snap package to snap-templates. Your snap applications depending on snap will continue to do so, but with a slightly lighter set of transitive dependencies. If you want to run snap init to generate a project template, you will now need to do cabal install snap-templates first instead of cabal install snap.

Migration Guide

  • Change your cabal files to depend on monad-control instead of MonadCatchIO-transformers.

  • Instead of deriving the MonadCatchIO type class, you should now make MonadBaseControl instances. Depending on your monad, this may require MonadBase and MonadTransControl instances as well. For examples of how to do that for common monad structures, look at Heist and snap (here, here, and here).

  • Any exception handling functions like try, catch, etc you were using from Control.Monad.CatchIO should now come from Control.Exception.Lifted which is provided by the lifted-base package.

  • initCookieSessionManager takes an additional Maybe ByteString argument representing an optional cookie domain. Passing Nothing as the new argument will give the same behavior as you had before.


The Snap Framework is composed of five major packages:

  • snap-core - A simple and stable web server API.

  • snap-server - A robust and well tested web server implementing the snap-core API.

  • heist - An HTML 5 template system allowing designers to make changes to markup without needing to have a Haskell toolchain installed and recompile the app.

  • snap - Umbrella project that integrates the above three packages, provides a snaplet system for building reusable web components, and includes built-in snaplets for common things like sessions, auth, templating, etc.

  • snap-templates - Provides an executable for generating Snap project templates.


We would like to thank the dozens of contributors who have helped over the years to get Snap to this milestone. Particular thanks go to Greg Hale who has been instrumental in getting us across the finish line for this release.