I am doing some additional experiments with Haskell and web development. In this post, I am looking at the problem of composability for web pages.

I have tested different methods for building a web page and what I don't like is the lack of composability. When I look at most web pages, I see several components with their own state and control flow. But, when I build a web page from those components, the composability on the server side (for the request handlers) is less obvious.

 

If you read my previous post, you'll be interested by this one. Indeed, I have discovered that the idea introduced in the previous post is working but not practical at all since it does not work with type synonym definitions. But, without synonyms, you can't really write the kind of types required to do http content-type comparison at the type level. By luck, some additional type hacking is solving the problem. Moreover, I think it can be useful to show in a very informal way a few applications of computations at type level. It is not so hard (but not totally trivial. I had to make several experiments to get it right).

 

In my last post, I was writing about the use of coproduct of free monads to do content-type dispatching in a web monad. It was working but it was not the right approach. I changed everything and introduced a lattice of lists at the type level to track hierarchical dependencies between formats and do content-type dispatching thanks to type information. I also added a few other features to my Web Monad.

 

From time to time, I continue my experiments with Haskell and web development and try to build my own framework. I will never release anything because I am just exploring a few ideas.

Recently, I experimented with a common problem : when a client is getting an URL, it is communicating the formats it wants with some preference values. The server should return the best format according to what is requested by the client and what is supported by the server.

 

I am pleased to release a new version of my PDF library for Haskell and an example showing what's possible with the library : a magazine generated in an automatic way with Haskell.

The parsing for the magazine has not yet been debugged totally and the image layout algorithm is currently very simple. The text formatting in columns is not always ok because I have not yet implemented an hyphenation algorithm. But, the result, still in beta, is good.