System.ReadEnvVar

This Haskell module exports functions for safely reading environment variables.

Documentation

readEnvVar

Arguments

:: Read a

=> String

environment variable to lookup

-> IO (Maybe a)

Lookup a value from an environment variable and read it in with readMaybe.

Note that this does not read string values as one would expect.

>>> import System.Environment (setEnv)
>>> setEnv "TEST_ENV_VAR" "2000"
>>> readEnvVar "TEST_ENV_VAR" :: IO (Maybe Int)
Just 2000
>>> readEnvVar "THIS_ENV_VAR_WILL_NOT_EXIST" :: IO (Maybe Int)
Nothing
>>> setEnv "TEST_ENV_VAR2" "some string 1"
>>> readEnvVar "TEST_ENV_VAR2" :: IO (Maybe String)
Nothing
>>> setEnv "TEST_ENV_VAR3" "\"some string 1\""
>>> readEnvVar "TEST_ENV_VAR3" :: IO (Maybe String)
Just "some string 1"

readEnvVarDef

Arguments

:: Read a

=> String

environment variable to lookup

-> a

default value to use if the environment variable either does not exist, or cannot be read

-> IO a

Lookup a value from an environment variable and read it in with readMaybe. If the environment variable doesn't exist, or it can't be read, use the default value.

Note that this does not read string values as one would expect.

>>> import System.Environment (setEnv)
>>> setEnv "TEST_ENV_VAR1" "1000"
>>> readEnvVarDef "TEST_ENV_VAR1" 5 :: IO Int
1000
>>> readEnvVarDef "THIS_ENV_VAR_WILL_NOT_EXIST" 5 :: IO Int
5
>>> setEnv "TEST_ENV_VAR2" "some string 1"
>>> readEnvVarDef "TEST_ENV_VAR2" "def val" :: IO String
"def val"
>>> setEnv "TEST_ENV_VAR3" "\"some string 1\""
>>> readEnvVarDef "TEST_ENV_VAR3" "def val" :: IO String
"some string 1"

lookupEnvDef

Arguments

:: IsString a

=> String

environment variable to lookup

-> a

default value to use if environment variable not defined

-> IO a

Like lookupEnv but take a default value.

>>> import System.Environment (setEnv)
>>> setEnv "TEST_ENV_VAR" "foo"
>>> lookupEnvDef "TEST_ENV_VAR" "bar" :: IO String
"foo"
>>> lookupEnvDef "THIS_ENV_VAR_WILL_NOT_EXIST" "bar" :: IO String
"bar"