data-default

A brilliant little package

Posted on 2020-04-04

It has taken me a while to properly appreciate data-default, but I am getting there. It is deceptively simple.

[Beware, Haddocks don’t always deal gracefully with packages re-exporting things from other packages. In this case the Default class is actually defined in data-default-class where you can see its definition clearly, unlike in data-default (currently).]

class Default a where
  def :: a

Default allows you to nominate defaults for your types, allowing them to be deployed with the drop of a def. If your type Foo supports generics you can let the class work out the default,

class Default Foo where

and use it in situations where a Foo is needed,

renderFoo :: Foo -> String

main :: Foo -> IO ()
main = putStrLn $ "the Foo : "++renderFoo def

or maybe:

main :: Foo -> IO ()
main = putStrLn $ "the Foo : "++show (def::Foo)

This works best where you have a type with an obvious canonical value, such as a configuration with an obvious default.

Got an issue with any of this? Please share or drop me a line (see below).