HAppS tutorial2

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Most of the stuff on this page refers to HAppS or greater. This is the most recent development version at the time of this writing. There is another tutorial for HAppS 0.8.8. Start there to get a background. This page is dedicated to the things that have changed in the newer versions.

Author's Note: This page will be in various states of disrepair for awhile. I'm creating this document as I am learning HAppS, so it may reflect my inaccurate perceptions. Some of the information on this page MAY EVEN BE WRONG. If you know it's wrong, I would be grateful for your help in improving it.

HAppS is a framework for developing Internet services quickly, deploying them easily, scaling them massively, and managing them effortlessly. Web, persistence, mail, DNS and database servers are all built-in so you can focus on app development rather than integrating and babysitting lots of different servers/services (the Haskell type system keeps everything consistent).


The basic web server framework in HAppS consists of a call to simpleHTTP that is passed a list of ServerParts that define the available pages. simpleHTTP searches through the ServerParts and returns the response generated by the first matching ServerPart. Typically ServerParts will match based on some part of the request URL.

The ServerPart is the workhorse of defining HAppS actions. The biggest goal of this document is to provide a useful guide to the ServerPart primitives that are available.

Basic ServerPart Functions

High-level description

Currently, many of these ServerPart primitives are defined in SimpleHTTP.hs and AlternativeHTTP.hs. AlternativeHTTP is being phased in to replace SimpleHTTP, so this section will focus on the definitions from AlternativeHTTP.

 * dir - match a directory at the front of the URL
 * path - another way to match a URL directory
 * multi - run a list of server parts
 * method - runs a server part if the request has the specified method
 * anyRequest - runs a server part no matter what the request was
 * withData
 * withDataFn
 * withRequest
 * applyRequest
 * ok
 * toResponse
 * fileServe - a generic file server that returns files from the local file system as responses


dir :: Monad m => String -> [ServerPartT m a] -> ServerPartT m a

The first argument is matched to the first directory element in the path URL. If it matches, then the ServerParts in the second argument are run.


path :: (FromReqURI a, Monad m) => (a -> [ServerPartT m r]) -> ServerPartT m r

Similar to dir except instead of passing a String and a list of ServerParts, you pass a function that returns a list of ServerParts.

FromReqURI is a type class that facilitates the conversion of data in the URI to Haskell types. The basic types are are already members of the class.


multi :: Monad m => [ServerPartT m a] -> ServerPartT m a

Returns a ServerPart that runs the list of ServerParts supplied in the first argument.

Error Codes

 * badRequest
 * unauthorizied
 * notFound
 * seeOther
 * found
 * movedPermanently
 * tempRedirect