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.

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.

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 defaultConfig

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.

If you need to have empty environment variables in your configuration, you can use something like the code below:

fromMaybe "" <$> lookupEnv "ENV_VAR"

Currently, dotenv-hs doesn't allow you to set empty environment variables, because of setEnv from our System.Environment. This is bug reported in GHC ticket. We have had many dicussions about this. Fortunately, there is already some work for this issue in GHC Phabricator.

In order to use compound env vars use the following sintax within your env vars ${your_env_var}. For instance:

DATABASE=postgres://${USER}@localhost/database

Running it on the CLI:

$ dotenv "echo $DATABASE"
postgres://myusername@localhost/database

In order to use the standard output of a command in your env vars use the following sintax $(your_command). For instance:

DATABASE=postgres://$(whoami)@localhost/database

Running it on the CLI:

$ dotenv "echo $DATABASE"
postgres://myusername@localhost/database

Env variables are simple strings. However, they can represent other types like integers, booleans, IP addresses, emails, URIs, and so on. We provide an interface that performs type checking after loading the envs and before running your application. If the type-check succeeded the application is executed, otherwise you will get an error with the types that mismatch.

In order to use this functionality you can use the loadSafeFile which takes the same configuration value as the loadFile function. Also, you need to have a .schema.yml in your current directory. This file must have the following structure:

- name: DOTENV
  type: bool
  required: true
- name: OTHERENV
  type: bool
- name: PORT
  type: integer
  required: true
- name: TOKEN
  type: text
  required: false

It is a list of type and envs. So, in this example, DOTENV must have a value of true or false otherwise it won't be parsed as a boolean value. And envs like PORT must be any integer. Currently, we are supporting the following types:

• bool - Accepts values false or true
• integer - Accepts values of possitive integers
• text - Any text

require specifies if the env var is obligatory or not. In case you set it to true but do not provide it, you wil get an exception. When required is omited, the default value is false.

NOTE: All the variables which are required in the schema.yml must be defined in the dotenvs.

The first argument to loadFile specifies the configuration. You cans use defaultConfig which parses the .env file in your current directory and doesn't override your envs. You can also define your own configuration with the Config type.

False in configOverride means Dotenv will respect already-defined variables, and True means Dotenv will overwrite already-defined variables.

In the configPath you can write a list of all the dotenv files where are envs defined (e.g [".env", ".tokens", ".public_keys"]).

In the configExamplePath you can write a list of all the dotenv example files where you can specify which envs must be defined until running a program (e.g [".env.example", ".tokens.example", ".public_keys.example"]). If you don't need this functionality you can set configExamplePath to an empty list.

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.

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

The -f flag is optional, by default it looks for the .env file in the current working directory.

$ dotenv myprogram

Aditionally you can pass arguments and flags to the program passed to Dotenv:

$ dotenv -f mydotenvfile myprogram -- --myflag myargument

or:

$ dotenv -f mydotenvfile "myprogram --myflag myargument"

Also, you can use a --example flag to use dotenv-safe functionality so that you can have a list of strict envs that should be defined in the environment or in your dotenv files before the execution of your program. For instance:

$ cat .env.example
DOTENV=
FOO=
BAR=

$ cat .env
DOTENV=123

$ echo $FOO
123

This will fail:

$ dotenv -f .env --example .env.example "myprogram --myflag myargument"
> dotenv: Missing env vars! Please, check (this/these) var(s) (is/are) set: BAR

This will succeed:

$ export BAR=123 # Or you can do something like: "echo 'BAR=123' >> .env"
$ dotenv -f .env --example .env.example "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.

The --schema FILE will get the envs configuration from the FILE. For instance:

$ cat .env
PORT=123a
$ cat .schema.yml
- name: PORT
  required: true
  type: integer

running dotenv will throw:

$ dotenv -s .schema.yml "echo $PORT"
dotenv: 1:4:
unexpected 'a'
expecting digit or end of input

NOTE: The flag can be omited when the .schema.yml is in the current working directory. To disable type checking add the flag --no-schema.

Justin Leitgeb

MIT

(C) 2015-2017 Stack Builders Inc.