Quick tutorial · Conferer

Simple configuration

Our Current app

You have a warp application which serves myApp like this:

main = do
  Warp.run myApp

Imports

Now we are here to configure things (hopefully without much work), so let's install the dependencies we need:

First conferer, which is the core library which provides the basic functions and typeclasses. To import conferer

import qualified Conferer

We also need to import conferer-warp which tells conferer how to create warp's Settings.

import Conferer.FromConfig.Warp ()

Note: From conferer-warp we only need some instances hence the ()

Creating a config

Now we need to do two things first create a Config, this object tells everything else where to look for configuration values. To create it call mkConfig "yourappname", use the your app's name there.

So our program now looks like this:

main = do
  config <- Conferer.mkConfig "awesomeapp"

Fetching values from the config

With our newly created config, we can use it to get a value of some type, in our example here we want to get a Warp.Settings from it, and we need to provide a default in case the user doesn't configure that.

To do that we do:

main = do
  config <- Conferer.mkConfig "awesomeapp"
  warpSettings <- Conferer.fetch config :: IO Warp.Settings

One of the mantras of conferer is that a program should work even if the user doesn provide configuration, so most of the time you want to provide a default.

Note: here we need to type this explicitly since warpSettings could be of many types that also can be fetched from a config. Once we start using it Haskell will be able to infer the type.

Letting Warp know about Conferer

Finally we have to tell warp to use our warpSettings, to do that we need to pass the Settings object we got to warp using Warp.runWithSettings (instead of Warp.run which doesn't allow using external configurations but is more convenient)

main = do
  config <- Conferer.mkConfig "awesomeapp"
  warpSettings <- Conferer.fetch config :: IO Warp.Settings
  Warp.runSettings warpSettings myApp

The configurable result

Now if I want to change the port of the app, I can change it by either:

Setting cli params like ./myApp --port=5555
Setting an environment variable called AWESOMEAPP_PORT=5555
In a config/development.properties file, you can have port=5555

But also we get many configurations that we usually can not touch like serverName (Server header that warp reports), timeout (maximum time in seconds for inactive clients), and 15 other warp configurations. If you want to know the options for a package you can take a look at its docs