Halendar - The Haskell Calendar

What

This is a (simplistic) calendar for the Functional Programming course at the Vrije Universiteit Brussel.

Requirements

Haskell
Cabal
SQLite 3
Library requirements are handled in the *.cabal file

Installation

Halendar uses Haskell (duh) and as such we rely on cabal. So start off by making sure you've got that installed.

Now, while being in the project folder, issue cabal install. This will start downloading and compiling a big amount of packages. Feel free to do something else while you wait. It will also create an executable in the following folder (on linux anyway): $HOME/.cabal/bin/. Add that to your path with PATH=$PATH:$HOME/.cabal/bin/. Finally, issue halendar to start running Halendar.

Caveat: from my (limited) experience, cabal is pretty horrible with properly managing all the dependencies of different projects. If you don't care about what packages you already had installed and you are having issues with running cabal install, see if removing ~/.cabal/ and ~/.ghc/ before trying it helps you any further.

Tables in the SQLite database will automatically be created when first run. If using this in production (why are you doing this?!), be sure to disable SQL query tracing in /snaplets/sqlite-simple/devel.cfg.

Understanding the code

This section is an overview of where to look for what in order to understand the code. Some working knowledge of Snap and Heist is required and outside the scope of this document.

Directories

snaplets/heist/templates/ are all the template files. Heist is the template engine. These files will be used to generate pages to serve to the user. Beyond being able to include one another, it's also possible to expose bind points to the haskell code (luckily!).
snaplets/sqlite-auth/ has configuration options for the Auth snaplet.
snaplets/sqlite-simple/ has configuration options for the sqlite database.
src/ holds the actual haskell code, more details to follow further on.
static/ holds files that can be served directly. For example images, CSS, ...

Time.hs

This file defines some functions that work on the UTCTime type from the Data.Time module from the time library. The repeat* ones take a tuple of two UTCTime and start tacking on the appropriate length, skipping entries that don't exist in another month/year. For example 29 February 2012 in either start of end of something repeating yearly would have as next result 29 February 2016 as there is no 29 February in 2013, 2014 and 2015. The get* ones are pretty obvious.

Db.hs

This one does three major things. First of all it defines the types we'll be using: an Event type and a User type that is a sort of simplified version of AuthUser from the Auth snaplet. Besides that it also provides functions to create the necessary database table as well as checking if it is already there. Finally there are functions that put an Event in the database as well as getting it back out.

Application.hs

This is pretty much some Snap configuring.

Main.hs

This is where the magic happens! At least that's what I keep telling myself.

On the one hand there's a bit to set up Snap completely. This includes setting up the routes from URLs to our handlers, setting up the snaplets that we are using and defining the main function.

On the other hand there's a boatload of handler functions for the different pages in the site as well as their helper functions. One of those helper functions worthy of some special attention is withLoggedInUser which is a wrapper function of sorts that

Checks if a user is logged in. If not, redirect to general "you need to log in" page
If logged in, simplify AuthUser to the Db.User we wrote, then pass that user on to the function which we are wrapping.

Now this wrapped function can be assured there is somoeone logged in as well as knowing who it is.

Something else to also take note of is that all the handlers of pages with a form are essentially two handlers in one. One of them triggers on a GET request and shows the form, the other triggers on a POST request and handles the submitted form.

ward / halendar