mmzk-env

mmzk-env is a library for reading environment variables into a user-defined data type. It provides a type-safe way to parse and validate environment variables, ensuring that they conform to the expected types.

Quick Start

module Data.Env where

import           Control.Monad.IO.Class
import           Data.Env.ExtractFields
import           Data.Env.RecordParser
import           GHC.Generics

-- | Example: Define an environment schema
data Config = Config
    { port     :: Int
    , name     :: String
    , mainHost :: String
    , debug    :: Maybe Bool }
    deriving (Show, Generic, EnvSchema)

-- | Run the validation
main :: IO ()
main = do
  errOrEnv <- validateEnv @Config
  case errOrEnv of
    Left err  -> putStrLn $ "Validation failed: " ++ err
    Right cfg -> putStrLn $ "Config loaded successfully: " ++ show cfg

With this setup, it requires the environment variables PORT, NAME, MAIN_HOST, and DEBUG to be set according to the types defined in the Config data type. The library will automatically parse these variables and validate them against the schema.

If any variable is missing or has an incorrect type, the validation will fail, and an error message will be printed.

Enum Support

The library also supports automatic parsing of enumerated types. You can define an enum and derive the TypeParser instance using the helper type EnumParser.

The extension DerivingVia is required for this feature.

{-# LANGUAGE DerivingVia #-}

data Gender = Male | Female
  deriving (Show, Eq, Enum, Bounded)
  deriving TypeParser via (EnumParser Gender)

print $ parseEnum @Gender "Male"   -- Right Male
print $ parseEnum @Gender "Female" -- Right Female