How to pass environment variables into Haskell programs run via stack?

Question

I just noticed that Haskell programs run via stack do not receive environment variables from the calling environment. Here's a sample program:

-- testenv.hs
import System.Environment
main :: IO ()
main = print =<< getEnv "FOOBAR"

If I run it without stack, like this, it works:

% FOOBAR=123 runhaskell testenv.hs
"123"

But using stack:

% FOOBAR=123 stack runhaskell testenv.hs
testenv.hs: FOOBAR: getEnv: does not exist (no environment variable)

Same goes for when it is compiled: FOOBAR=123 stack exec testenv fails while FOOBAR=123 .stack-work/install/BLAHBLAH/testenv works.

Is there a way to force stack to pass-through certain environment variables?

The real problem I'm having is with yesod devel, there are some settings I want to override with environment variables, but yesod devel uses stack to run the program so they don't go through.

This is stack 1.6.5 on NixOS 18.03.132262.0a73111bc29.

Answer 1

It seems like this is the relevant section of the stack manual, which I missed:

“By default, stack will run the build in a pure Nix build environment (or shell), which means two important things: (1) basically no environment variable will be forwarded from your user session to the nix-shell [...]”

So this advice worked:

“To override this behaviour, add pure: false to your stack.yaml or pass the --no-nix-pure option to the command line.”

% FOOBAR=123 stack --no-nix-pure runhaskell testenv.hs
"123"