24 Days of GHC Extensions: Nullary Type Classes

While I wait for a few more guest posts to soldify, I thought it might be fun to look at another new extension to GHC - NullaryTypeClasses.

{-# LANGUAGE NullaryTypeClasses #-}

Ordinarily in Haskell, a type class is defined as the following:

class ctx => C a where

That is, our type class can have a context, and can operate on a single type parameter. However, working with one type is a restriction - later, we’ll see about working with multiple type parameters is there any reason we couldn’t work on no type parameters?

It sounds crazy, but GHC can do just that! The linked documentation has one scenario of when you may want to do that - if you need to make a controversial assumption, by using a nullary type class you can give your end users the choice as to whether or not they want to commit to using code that requires that assumption to hold.

However, nullary type classes can also be used as a form of implicit configuration. For example, let’s assume we’re building a Haskell library, and we’d like to give our users the ability to log its activity. There are many ways to log messages these days - in development we might use STDERR, but in production we might push our logs directly to journald or syslog.

By using nullary type classes, we can abstract over this logging function, and let the end user provide a logging strategy. To begin, we define our type class, mostly as normal - but we don’t use any type parameters:

class Logger where
  logMessage :: String -> IO ()

The presence of this type class allows us to call logMessage, but defer the implementation to the user of the library:

type Present = String
queueNewChristmasPresents :: Logger => [Present] -> IO ()
queueNewChristmasPresents presents = do
  mapM (logMessage . ("Queueing present for delivery: " ++)) presents
  return ()

Notice how our library function clearly indicates the assumption - here we are assuming that it will be possible to log a message, using IO.

Now to provide a logging implementation, all we have to do is provide the instance for the Logger type class:

instance Logger where
  logMessage t = putStrLn ("[XMAS LOG]: " ++ t)

Running our library function does just what we would expect:

.> queueNewChristmasPresents ["Cuddly Lambda", "Gamma Christmas Pudding"]
[XMAS LOG]: Queueing present for delivery: Cuddly Lambda
[XMAS LOG]: Queueing present for delivery: Gamma Christmas Pudding


If you’re feeling a little uncomfortable now - I’m with you. It’s not clear to me whether this is really a good idea or not, but it’s nice that it is available in GHC. In a future post we’ll look at another extension that attempts to solve the implicit configuration problem.

This post is part of 24 Days of GHC Extensions - for more posts like this, check out the calendar.

You can contact me via email at ollie@ocharles.org.uk or tweet to me @acid2. I share almost all of my work at GitHub. This post is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.