Env.hs

{-# options_ghc -Wall -Werror #-}
module Env where

import Data.Maybe (fromMaybe)
import qualified System.Environment

data LogLevel = DEBUG | INFO | WARN | ERROR
  deriving (Show, Read)

newtype Parser a = Parser { parse :: String -> IO a }

filePath :: Parser FilePath
filePath = Parser pure

logLevel :: Parser LogLevel
logLevel = Parser (pure . read)

required :: String -> Parser a -> IO a
required key parser = do
  value <- System.Environment.lookupEnv key
  case value of
    Nothing -> error $ "missing required key: " <> show key
    Just input -> parse parser input

optional :: String -> Parser a -> IO (Maybe a)
optional key parser = do
  value <- System.Environment.lookupEnv key
  case value of
    Nothing -> pure Nothing
    Just input -> Just <$> parse parser input

withDefault :: Functor f => f (Maybe a) -> a -> f a
withDefault ma a = fromMaybe a <$> ma

infixl 5 `withDefault`

data Config 
  = Config 
  { _HOMEPAGE_CONFIG_FILE :: FilePath
  , _HOMEPAGE_LOG_FILE :: Maybe FilePath
  , _HOMEPAGE_LOG_LEVEL :: LogLevel
  } deriving Show

main :: IO ()
main = do
  config <-
    Config <$>
      optional "HOMEPAGE_CONFIG_FILE" filePath `withDefault` "./homepage.json" <*>
      optional "HOMEPAGE_LOG_FILE" filePath <*>
      optional "HOMEPAGE_LOG_LEVEL" logLevel `withDefault` DEBUG
  print config