Cfg.Env

This module contains all the functions for interacting with environment variables as a configuration source.

Since: 0.0.2.0

Retrieval Functions

envSourceSep

Arguments

:: MonadIO m

=> Text

Separator

-> KeyTree Text Text

Configuration source

-> m (KeyTree Text Text)

Configuration tree with values filled in

This function folds the tree from root to leaf accumulating the keys along the way. At the leaf we lookup the aggregated key in the environment, if there is a default then we use that for missing keys.

If you are looking at the source code this is what the functions in the where clause do:

• valF: Gets called on Pure values in the original tree passed in, these indicate defaulted values, so we use the default if looking the value up in the environment failed.
• accF: This operates on the accumulated key, and is responsible for looking up the value in the environment when we hit the Free M.empty case.
• stepF: This is the step function for the fold and accumulates the keys as we traverse down the tree.
• mkKey: This function is the same as getEnvKey except that it uses flip mappend. The reason for this is that we insert keys into the accumulator as we traverse down so they end up in reversed order, then we foldr over them so we just need to make sure that we are placing the elements at the end of the list on the left hand side of the aggregate key.

Since: 0.0.1.0

envSource :: (MonadFail m, MonadIO m) => KeyTree Text Text -> m (KeyTree Text Text)

This function is the same as envSourceSep but with the separator specialized to "_".

>>> import System.Environment
>>> import Data.Map qualified as M
>>> setEnv "A_B" "Functor"
>>> setEnv "A_C" "Applicative"
>>> setEnv "A_D" "Monad"
>>> envSource $ Free $ M.fromList [("A", Free $ M.fromList [("B", Free M.empty), ("C", Free M.empty), ("D", Free M.empty)])]
Free (fromList [("A",Free (fromList [("B",Pure "Functor"),("C",Pure "Applicative"),("D",Pure "Monad")]))])

Since: 0.0.1.0

getEnvConfigSep

Arguments

:: forall a m. (MonadFail m, MonadIO m, ConfigSource a, ConfigParser a)

=> Text

Separator

-> m (Either ConfigParseError a)

Requires a type annotation for your configuration type (with a ConfigSource and ConfigParser instance), and a separator, and will go out and fetch the values from environment variables then return your type parsed from those values.

getEnvConfigSep @AppConfig "_"

Since: 0.0.1.0

getEnvConfig :: forall a m. (MonadFail m, MonadIO m, ConfigSource a, ConfigParser a) => m (Either ConfigParseError a)

The same as getEnvConfigSep but with the separator hard coded to "_"

Since: 0.0.1.0

Printing Functions

printDotEnv'

Arguments

:: FilePath

Destination filepath

-> Text

Separator

-> KeyTree Text Text

Source representation

-> IO ()

This function can be used to print a dotenv style file with all the aggregate keys, none of the values will be filled in.

Useful for testing what your expected environment variables should look like, and generating an env var file template.

Since: 0.0.1.0

printDotEnv :: forall {k} (a :: k). ConfigSource a => FilePath -> IO ()

The same as printDotEnv' but with the separator hard coded to "_" and it uses a type application to generate the configuration source tree representation.

printDotEnv @AppConfig ".env"

Since: 0.0.1.0