Config is a small and typesafe configuration library. It provides good error messages and comes with a bottom-up typechecker in order to catch more configuration errors.

Here some use-cases:

>>> foo = ["bar", { baz : 42 }]
foo:1:8-13: Expecting String but having Object

Reason: List has only one inner type

>>> foo = ["bar"] [{ baz : 42 }]
foo:1:7-14: Expecting List[String] but having List[Object]

Reason: You can't merge Lists of different types

It uses Typesafe-config format: HOCON. HOCON stands for Human-Optimized Config Object Notation. It's basically a JSON superset

Here's an example:

-- app.conf
# This is a comment

foo.bar = ${toto}

toto = false

rawString = """
            This is a multi-
            lines String
            """

another.string = "I'm a String"

one.more.string = one more string

nested {
   list: [ one
         , 1
         , "both"]

   homing = {
     pass: { b: feez } { a: "Prop"}
   }

   another: [1,2,3] [4,5,6]
}

How to use:

-- Example.hs
{--}
import Data.Config
import Data.Text (Text)

data Foo = Foo { fooPort :: Int, fooAddr :: Text }

main :: IO ()
main = do
  foo <- loadFooProps
  withFoo foo

  where
    loadFooProps = do
      config <- loadConfig "conf/baz.conf"
      port   <- getInteger "foo.port" config
      addr   <- getString "foo.addr" config
      return (Foo port addr)

withFoo :: Foo -> IO ()
withFoo = ...