[Haskell-beginners] How to define macro var with stack/cabal/etc?

[PY]

Hello all,

I'm using conditional compilation:

#ifdef BLAH

blahBlah

#endif

And I want to define BLAN, but no in *.hs file. For example, with 
`stack` option, in yaml file, in cabal file, in environment variable... 
How can I do it? For example, `stack -DBLAH=1 build` or something else. 
What are the options?

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Best regards, Paul