Dotenv files for Haskell
In most applications,
configuration should be separated from code. While
it usually works well to keep configuration in the environment, there
are cases where you may want to store configuration in a file outside
of version control.
“Dotenv” files have become popular for storing configuration,
especially in development and test environments. In
Ruby,
Python and
Javascript there are libraries
to facilitate loading of configuration options from configuration
files. This library loads configuration to environment variables for
programs written in Haskell.
Installation
In most cases you will just add dotenv
to your cabal file. You can
also install the library and executable by invoking stack install dotenv
.
Usage
Set configuration variables in a file following the format below:
S3_BUCKET=YOURS3BUCKET
SECRET_KEY=YOURSECRETKEYGOESHERE
Then, calling Dotenv.load
from your Haskell program reads the above
settings into the environment:
import qualified Configuration.Dotenv as Dotenv
Dotenv.loadFile False "/path/to/your/file"
After calling Dotenv.load
, you are able to read the values set in your
environment using standard functions from System.Environment
such as
lookupEnv
and getEnv
.
Configuration
The first argument to loadFile
specifies whether you want to
override system settings. False
means Dotenv will respect
already-defined variables, and True
means Dotenv will overwrite
already-defined variables.
Advanced Dotenv File Syntax
You can add comments to your Dotenv file, on separate lines or after
values. Values can be wrapped in single or double quotes. Multi-line
values can be specified by wrapping the value in double-quotes, and
using the “\n” character to represent newlines.
The spec file is the best
place to understand the nuances of Dotenv file parsing.
Command-Line Usage
You can call dotenv from the command line in order to load settings
from one or more dotenv file before invoking an executable:
dotenv -f mydotenvfile myprogram
Aditionally you can pass arguments and flags to the program passed to
Dotenv:
dotenv -f mydotenvfile myprogram -- --myflag myargument
Hint: The env
program in most Unix-like environments prints out the
current environment settings. By invoking the program env
in place
of myprogram
above you can see what the environment will look like
after evaluating multiple Dotenv files.
Author
Justin Leitgeb
License
MIT
Copyright
(C) 2015-2017 Stack Builders Inc.