What is the reason not to use stack -nix when using nix?

Question

I am on NixOS, but I think this question should apply to any platform using nix.

I found that stackyou can use it with pairs of options to create a project, but I don’t quite understand the differences between them,

• stack
• stack --system-ghc
• stack --nix

Question :

• If I use nix (NixOS in my case), is there some reason why I don't want to use the argument --nix?
• What is the nixway to solve the haskell project instead of cabalusing cabal(cabal2nix)?
• I found that the stack restores many libraries that are already installed nix, what is the reason for this?