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Monday, November 26, 2012

Building a Haskell web app with Snap: Snap quickstart guide

I'm a Haskell newbie, and it's fun. I thought I'd document my adventures. Starting with my attempt to build a web app.

I could have spent months picking a web framework. There are decent comparisons on the web. I picked Snap, because of the name (and perhaps some advice from a geeky friend of mine). I've written web frameworks (in Python) and the intro to Snap caught my eye. All pretty arbitrary reasons, so let's get started.

First there is a quickstart guide. Great, who doesn't love quickstart guides? It tells me how to install the framework, though that redirects me to another page, but I don't mind. I would have to have Cabal installed, otherwise it wouldn't work. Instead of just plain:

cabal install snap

I do:

cabal install --user --prefix=$HOME snap

Which seems the most convenient way to put things in my ~ tree rather than anywhere on the system. Great it works first time with no dependency issues, conflicts, or compile errors. That's pretty rare, so great start!

I start to follow the quickstart guide. First create a directory, then call:

snap init barebones

This is great too, so we have a utility script to perform basic operations (as any decent web framework should), and by the look of "barebones" it seems that there are multiple possible templates to start from. Another great feature. The guide suggests running

snap init -h

To see the list of templates, and here they are:

snap init [type]

    [type] can be one of:
      default   - A default project using snaplets and heist
      barebones - A barebones project with minimal dependencies
      tutorial  - The literate Haskell tutorial project

So three starter templates, one of which is a tutorial. So, two starter templates: "barebones" and "default". I guess we will use default in the future when writing a real app, with Snaplets. I won't have much cause to write my own templates, I hope, so moving on.

The guide doesn't explain much what it created, but my tree looks like this now:


Pretty much nothing there except Main.hs, which looks like this:

{-# LANGUAGE OverloadedStrings #-}
module Main where

import           Control.Applicative
import           Snap.Core
import           Snap.Util.FileServe
import           Snap.Http.Server

main :: IO ()
main = quickHttpServe site

site :: Snap ()
site =
    ifTop (writeBS "hello world") <|>
    route [ ("foo", writeBS "bar")
          , ("echo/:echoparam", echoHandler)
          ] <|>
    dir "static" (serveDirectory ".")

echoHandler :: Snap ()
echoHandler = do
    param <- getParam "echoparam"
    maybe (writeBS "must specify echo/param in URL")
          writeBS param

I don't understand this code exactly, but like a lot of things in Haskell, I just don't have to (yet! I imagine one day I will), I can just copy the style. It reads nicely though.

  • Some imports
  • A main function that serves the site
  • A site function that returns some Snap, Snap looking like a monad. This is the part that I don't exactly get: <|> anyone?. We are obviously defining routes, and I am guessing there are 3 ways of hitting this site (This is possibly a bit too much information for a quickstart, I might be happier with just the echo handler, but maybe that isn't possible on its own. Doesn't matter though):
    • / (ifTop, I'm guessing for the root) where we just return "hello world"
    • /foo which just returns "bar"
    • /echo/<something> which echoes the something back at us
  • The echo handler itself, which gets the parameter defined in the route and returns it. Not exactly sure what the maybe line is doing here. I guess it is a failure condition, though I am more used to frameworks where a missing echoparam would not match the route and just 404.

Because the snap utility created a cabal file for us (how convenient), I can just install the app with:

cabal install --user --prefix=$HOME

That's right, my app is an executable, now installed, which I can just run. That's pretty awesome, we don't have stuff like that in Python frameworks. I had called my initial directory snaptest and that is how the executable comes out. Let's run it:

$ snaptest -p 8000
Listening on

And here is what we get:

hello world


No handler accepted echo


Great, as we mostly imagined, except I can't make the maybe condition in the echo handler happen, but I don't care much either. After maybe 256 seconds of actual effort and 16 minutes after deciding to write a Haskell web app, I have one.

This is the best quick start experience I have had for any web application framework in any language, so things bode well for the future.