nix-env and nix-shell have different versions of snap - haskell

Question

I was following directions on reflex-platfrom project development,trying to test servant-reflex as a submodule.
My project is here.
In my backend.cabal, I have a built-depend:
snap >= 1.1.1.0 && < 1.2
When I nix-shell -A shells.ghc --run "cabal new-build all", it tries to install heist-1.0.1.0 and snap-1.0.0.2, then failed at
Configuring heist-1.0.1.0...
Setup: Encountered missing dependencies:
aeson >=0.6 && <1.2
To see what in my nixos-unstable, I:
`nix-channel --list`
nixos https://nixos.org/channels/nixos-unstable
`nix-env -f "<nixpkgs>" -qaP -A haskellPackages.aeson`
warning: Nix search path entry '/home/demo/.nix-defexpr/channels' does not exist, ignoring
haskellPackages.aeson aeson-1.2.4.0
`nix-env -f "<nixpkgs>" -qaP -A haskellPackages.snap`
warning: Nix search path entry '/home/demo/.nix-defexpr/channels' does not exist, ignoring
haskellPackages.snap snap-1.1.0.0
`nix-env -f "<nixpkgs>" -qaP -A haskellPackages.heist`
warning: Nix search path entry '/home/demo/.nix-defexpr/channels' does not exist, ignoring
haskellPackages.heist heist-1.0.1.2
Q: Why does nix-shell install heist-1.0.1.0 and snap-1.0.0.2, instead of heist-1.0.1.2 and snap-1.1.0.0, which then can dependent on aeson-1.2.4.0?

Answer 1

Got an answer from elvishjerricco on IRC #nixos.
To doJailbreak heist, you'd use the overrides argument to
project
packages is for just declaring directories that you want to turn
into haskell packages; it'll run cabal2nix for you. overrides is for
doing derivation changes to the haskell package set.
default.nix
(import ./reflex-platform {}).project ({ pkgs, ... }: {
overrides = self: super: {
heist = pkgs.haskell.lib.doJailbreak super.heist;
map-syntax = pkgs.haskell.lib.doJailbreak super.map-syntax;
};
packages = {
common = ./common;
backend = ./backend;
frontend = ./frontend;
google-maps-reflex = ./google-maps-reflex;
};
shells = {
ghc = ["common" "backend" "frontend" "heist"]; # "backend" "frontend"];
ghcjs = ["common" "frontend"];
};
})