Let's face it, dealing with environment variables in Haskell isn't that satisfying.
import System.Environment
import Data.Text (pack)
import Text.Read (readMaybe)
data ConnectInfo = ConnectInfo {
pgPort :: Int
pgURL :: Text
} deriving (Show, Eq)
getPGPort :: IO ConnectInfo
getPGPort = do
portResult <- lookupEnv "PG_PORT"
urlResult <- lookupEnv "PG_URL"
case (portResult, urlResult) of
(Just port, Just url) ->
case readMaybe port :: Maybe Int of
Nothing -> error "PG_PORT isn't a number"
Just portNum -> return $ ConnectInfo portNum (pack url)
(Nothing, _) -> error "Couldn't find PG_PORT"
(_, Nothing) -> error "Couldn't find PG_URL"
-- Pretty gross right...Another attempt to remedy the lookup madness is with a MaybeT IO a. See below.
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
import Control.Applicative
import Control.Monad.Trans.Maybe
import Control.Monad.IO.Class
import System.Environment
newtype Env a = Env { unEnv :: MaybeT IO a }
deriving (Functor, Applicative, Monad, MonadIO, Alternative, MonadPlus)
getEnv :: Env a -> IO (Maybe a)
getEnv env = runMaybeT (unEnv env)
env :: String -> Env a
env key = Env (MaybeT (lookupEnv key))
connectInfo :: Env ConnectInfo
connectInfo = ConnectInfo
<$> env "PG_HOST"
<*> env "PG_PORT"
<*> env "PG_USER"
<*> env "PG_PASS"
<*> env "PG_DB"This abstraction falls short in two areas:
- Lookups don't return any information when a variable doesn't exist (just a
Nothing) - Lookups don't attempt to parse the returned type into something meaningful (everything is returned as a
StringbecauselookupEnv :: String -> IO (Maybe String))
What if we could apply aeson's FromJSON / ToJSON pattern to give us variable lookups that provide both key-lookup and parse failure information?
Armed with the GeneralizedNewTypeDeriving extension we can derive instances of Var that will parse to and from an environment variable. The Var typeclass is simply:
class Var a where
toVar :: a -> String
fromVar :: String -> Maybe aWith instances for most concrete and primitive types supported (Word8 - Word64, Int, Integer, String, Text, etc.) the Var class is easily deriveable. The FromEnv typeclass provides a parser type that is an instance of MonadError String and MonadIO. This allows for connection pool initialization inside of our environment parser and custom error handling. The ToEnv class allows us to create an environment configuration given any a. See below for an example.
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE RecordWildCards #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE DeriveDataTypeable #-}
------------------------------------------------------------------------------
module Main ( main ) where
------------------------------------------------------------------------------
import Control.Applicative
import Control.Exception
import Control.Monad
import Data.Either
import Data.Word
import System.Environment
import System.Envy
------------------------------------------------------------------------------
data ConnectInfo = ConnectInfo {
pgHost :: String
, pgPort :: Word16
, pgUser :: String
, pgPass :: String
, pgDB :: String
} deriving (Show)
------------------------------------------------------------------------------
-- | FromEnv instances support popular aeson combinators *and* IO
-- for dealing with connection pool initialization. `env` is equivalent to (.:) in `aeson`
-- and `envMaybe` is equivalent to (.:?), except here the lookups are impure.
instance FromEnv ConnectInfo where
fromEnv _ =
ConnectInfo <$> envMaybe "PG_HOST" .!= "localhost"
<*> env "PG_PORT"
<*> env "PG_USER"
<*> env "PG_PASS"
<*> env "PG_DB"
------------------------------------------------------------------------------
-- | To Environment Instances
-- (.=) is a smart constructor for producing types of `EnvVar` (which ensures
-- that Strings are set properly in an environment so they can be parsed properly
instance ToEnv ConnectInfo where
toEnv ConnectInfo {..} = makeEnv
[ "PG_HOST" .= pgHost
, "PG_PORT" .= pgPort
, "PG_USER" .= pgUser
, "PG_PASS" .= pgPass
, "PG_DB" .= pgDB
]
------------------------------------------------------------------------------
-- | Example
main :: IO ()
main = do
setEnvironment (toEnv :: EnvList ConnectInfo)
print =<< do decodeEnv :: IO (Either String ConnectInfo)
-- unsetEnvironment (toEnv :: EnvList ConnectInfo) -- remove when doneOur parser might also make use a set of an optional default values provided by the user, for dealing with errors when reading from the environment
instance FromEnv ConnectInfo where
fromEnv Nothing =
ConnectInfo <$> envMaybe "PG_HOST" .!= "localhost"
<*> env "PG_PORT"
<*> env "PG_USER"
<*> env "PG_PASS"
<*> env "PG_DB"
fromEnv (Just def) =
ConnectInfo <$> envMaybe "PG_HOST" .!= (pgHost def)
<*> envMaybe "PG_PORT" .!= (pgPort def)
<*> env "PG_USER" .!= (pgUser def)
<*> env "PG_PASS" .!= (pgPass def)
<*> env "PG_DB" .!= (pgDB def)Note: As of base 4.7 setEnv and getEnv throw an IOException if a = is present in an environment. envy catches these synchronous exceptions and delivers them
purely to the end user.
As of version 1.0, all FromEnv instance boilerplate can be completely removed thanks to GHC.Generics! Below is an example.
{-# LANGUAGE DeriveGeneric #-}
module Main where
import System.Envy
import GHC.Generics
import System.Environment.Blank
-- This record corresponds to our environment, where the field names become the variable names, and the values the environment variable value
data PGConfig = PGConfig {
pgHost :: String -- "PG_HOST"
, pgPort :: Int -- "PG_PORT"
} deriving (Generic, Show)
instance FromEnv PGConfig
-- Generically creates instance for retrieving environment variables (PG_HOST, PG_PORT)
main :: IO ()
main = do
_ <- setEnv "PG_HOST" "valueFromEnv" True
_ <- setEnv "PG_PORT" "66354651" True
print =<< do decodeEnv :: IO (Either String PGConfig)
-- > PGConfig { pgHost = "valueFromEnv", pgPort = 66354651 }If the variables are not found in the environment, the parser will currently fail with an error about the first missing field.
The user can decide to provide a default value, whose fields will be used by the generic instance, if retrieving them from the environment fails.
defConfig :: PGConfig
defConfig = PGConfig "localhost" 5432
main :: IO ()
main =
_ <- setEnv "PG_HOST" "customURL" True
print =<< decodeWithDefaults defConfig :: IO (Either String PGConfig)
-- > PGConfig { pgHost = "customURL", pgPort = 5432 }Suppose you'd like to customize the field name (i.e. add your own prefix, or drop the existing record prefix). This too is possible. See below.
{-# LANGUAGE DeriveGeneric #-}
module Main where
import System.Envy
import GHC.Generics
data PGConfig = PGConfig {
connectHost :: String -- "PG_HOST"
, connectPort :: Int -- "PG_PORT"
} deriving (Generic, Show)
instance DefConfig PGConfig where
defConfig = PGConfig "localhost" 5432
-- All fields will be converted to uppercase
instance FromEnv PGConfig where
fromEnv = gFromEnvCustom Option {
dropPrefixCount = 7
, customPrefix = "CUSTOM"
}
main :: IO ()
main =
_ <- setEnv "CUSTOM_HOST" "customUrl" True
print =<< do decodeEnv :: IO (Either String PGConfig)
-- PGConfig { pgHost = "customUrl", pgPort = 5432 }It's also possible to avoid typeclasses altogether using runEnv with gFromEnvCustom.
{-# LANGUAGE DeriveGeneric #-}
module Main where
import System.Envy
import GHC.Generics
data PGConfig = PGConfig {
connectHost :: String -- "PG_HOST"
, connectPort :: Int -- "PG_PORT"
} deriving (Generic, Show)
-- All fields will be converted to uppercase
getPGEnv :: IO (Either String PGConfig)
getPGEnv = runEnv $ gFromEnvCustom defOption
(Just (PGConfig "localhost" 5432))
main :: IO ()
main = print =<< getPGEnv
-- PGConfig { pgHost = "localhost", pgPort = 5432 }