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.

Contents

• mmzk-env
  • Contents
  • Quick Start
    • Custom Environment Variable Mapping
  • Enum Support
  • Witness Types: Avoiding Newtype Boilerplate
    • The Problem: Newtype Boilerplate
    • The Solution: Witnesses
    • Key Benefits
    • Available Witnesses

Quick Start

Full example →

{-# LANGUAGE DeriveAnyClass #-} {-# LANGUAGE DeriveGeneric #-} {-# LANGUAGE TypeApplications #-} import Data.Env 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.

Custom Environment Variable Mapping

Full example →

By default, the library converts camelCase field names to UPPER_SNAKE_CASE (e.g., mainHost → MAIN_HOST).

If you want to use uppercase environment variable names without underscores (like MAINHOST instead of MAIN_HOST), you can use validateEnvWith (or validateEnvWWith for witness types) with a custom mapping function:

data Config = Config { port :: Int , name :: String , main_host :: String -- Will map to "MAINHOST" with custom mapping , debug :: Maybe Bool } deriving (Show, Generic, EnvSchema) main :: IO () main = do errOrEnv <- validateEnvWith @Config (map toUpper) case errOrEnv of Left err -> putStrLn $ "Validation failed: " ++ err Right cfg -> putStrLn $ "Config loaded successfully: " ++ show cfg 

With validateEnvWith (map toUpper), the field main_host will look for the environment variable MAINHOST instead of MAIN_HOST.

You can provide any custom mapping function to validateEnvWith to transform field names to environment variable names according to your needs.

Enum Support

Full example →

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 #-} {-# LANGUAGE TypeApplications #-} 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 

Witness Types: Avoiding Newtype Boilerplate

The library provides a "witness" pattern that allows you to enhance parsing behaviour without wrapping values in newtypes. This is useful when you need features like default values, validation, or transformation but want to keep your final data types simple.

The Problem: Newtype Boilerplate

Full example →

Let's say you want to parse a PostgreSQL port that defaults to 5432. Without witnesses, you might create a newtype wrapper:

{-# LANGUAGE DeriveAnyClass #-} {-# LANGUAGE DeriveGeneric #-} {-# LANGUAGE TypeApplications #-} import Data.Env import Data.Env.TypeParser import Data.Word import GHC.Generics -- Define a newtype wrapper for the port newtype PsqlPort = PsqlPort Word16 deriving (Show, Eq) -- Implement custom parsing with default value instance TypeParser PsqlPort where parseType "" = Right (PsqlPort 5432) -- Default to 5432 parseType str = case parseType str of Right port -> Right (PsqlPort port) Left err -> Left err data Config = Config { psqlPort :: PsqlPort , dbName :: String } deriving (Show, Generic, EnvSchema) 

Now when you use your config, you have to constantly unwrap the value:

unpackPort :: PsqlPort -> Word16 unpackPort (PsqlPort port) = port connectToDatabase :: Config -> IO Connection connectToDatabase cfg = connect $ defaultConnectInfo { connectPort = unpackPort (psqlPort cfg) -- Annoying unpacking! , connectDatabase = dbName cfg } where unpackPort (PsqlPort port) = port 

The Solution: Witnesses

Full example →

With witness types, you can specify parsing behaviour at the type level while keeping the final value unwrapped:

{-# LANGUAGE DataKinds #-} {-# LANGUAGE DeriveGeneric #-} {-# LANGUAGE DerivingVia #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE StandaloneDeriving #-} {-# LANGUAGE TypeApplications #-} import Data.Env import Data.Env.Witness.DefaultNum import Data.Word import GHC.Generics import Data.Env.TypeParserW data Config c = Config { psqlPort :: Column c (DefaultNum 5432 Word16) Word16 -- Defaults to 5432 , dbName :: Column c (Solo String) String } deriving (Generic, EnvSchemaW) instance EnvSchemaW (Config \'Dec) deriving stock instance Show (Config 'Res) -- For printing the result -- Validate environment variables with defaults main :: IO () main = do errOrConfig <- validateEnvW @(Config 'Dec) case errOrConfig of Left err -> putStrLn $ "Validation failed: " ++ err Right cfg -> connectToDatabase cfg -- cfg :: Config 'Res 

The magic happens with the Column type family and the ColumnType phantom type:

• Config 'Dec (Declaration): The type used for parsing, where each field is (witness, value)
  • This works under the hood for the generic instances and users typically don't interact with it directly
• Config 'Res (Result): The type you work with, where each field is just value
• Column c witness a: Expands to (witness, a) when c = 'Dec, or just a when c = 'Res

Now your final config has no wrappers:

connectToDatabase :: Config 'Res -> IO Connection connectToDatabase cfg = connect $ defaultConnectInfo { connectPort = psqlPort cfg -- Direct access to Word16! , connectDatabase = dbName cfg } 

Key Benefits

1. No Unpacking: Your final data type contains raw values (Word16, String, etc.), not newtypes
2. Type-Level Defaults: Default values are specified in the type signature using type-level naturals
3. Flexible Parsing: Different witness types provide different parsing strategies (defaults, validation, transformation)

Available Witnesses

• Solo a: Standard parsing without special behaviour (equivalent to TypeParser)
• DefaultNum n a: Numeric types with a type-level default value n
• • DefaultString s a: String types with a type-level default value s
• Custom witnesses: You can define your own by implementing the TypeParserW class

More built-in witnesses will be provided.

For more complex parsing needs, witnesses provide a way to augment behaviour without polluting your domain types with wrapper noise.