Environment Variables in Haskell

In this post we will go over how to accept environmental variables in Haskell.

import System.Environment

main :: IO ()
main = do

We can compile this if we put it in a file called env.hs.

ghc env.hs

and run it with an ad-hoc ENV variable:

myvariable="what" ./env

Which will print out:

"what"

but this will throw an error if the ENV var doesn't exist:

λ ./env
env: myvariable: getEnv: does not exist (no environment variable)

To solve that issue we can use lookupEnv with fromMaybe

import System.Environment (lookupEnv)
import Data.Maybe (fromMaybe)

main :: IO ()
main = do

which will give us a default value when the ENV variable doesn't exist:

λ ./env
"mydefaultvalue"
λ myvariable="what" ./env
"what"