Please refer to haskell-nix for the complete guide, this is a stripped down version.

Globally install cabal:

nix-env --install cabal-install
nix-env --install cabal2nix

Init cabal project without install ghc globally, cabal will pull in ghc afterwards:

nix-shell --packages ghc --run 'cabal init'

Add default.nix, note we have added ghcid:

{ pkgs ? import <nixpkgs> { } }:
pkgs.mkShell {
  buildInputs = [ pkgs.haskellPackages.ghcid ];
}

Now run

nix-shell

we are now inside nix-shell, create project.nix:

cabal2nix . > project.nix

change default.nix to

{ pkgs ? import <nixpkgs> { } }:
let
  project = pkgs.haskellPackages.callPackage ./project.nix { };
in pkgs.mkShell {
  buildInputs = [ pkgs.haskellPackages.ghcid ];
  inputsFrom = [ project.env ];
}

exit nix-shell (ctrl-D) and nix-shell enter again, run

ghcid "--command=ghci Main.hs" --test=":main"

and we are good to go, modifying Main.hs will rerun it.