Archived: this project is no longer seeing updates. I (highly) recommend you migrate to dotenv-hs, which has the same usage, the same features, and much more.
This is effectively a port of dotenv, whose README explains it best:
Storing configuration in the environment is one of the tenets of a twelve-factor app. Anything that is likely to change between deployment environments–such as resource handles for databases or credentials for external services–should be extracted from the code into environment variables.
But it is not always practical to set environment variables on development machines or continuous integration servers where multiple projects are run. dotenv loads variables from a .env file into ENV when the environment is bootstrapped.
This library exposes functions for doing just that.
import LoadEnv
import System.Environment (lookupEnv)
main :: IO ()
main = do
loadEnv
print =<< lookupEnv "FOO"% cat .env
FOO=bar
% runhaskell main.hs
Just "bar"stack setup
stack build --pedantic --test