Nix: adding other derivation's subdirectory to PATH - nix

Question

I want to make a foo.nix file that when you nix-shell foo.nix and enter the sand-boxed shell, the PATH includes some other derivation's bin/ subdirectory. How can I do that?
--- minimal reproducible example ---
Following is my best attempt so far.
This is my current .nix file
let git = builtins.fetchGit {
url = https://github.com/ingun37/haskell-script.git;
rev = "e96ac79d6bf338c98697bb4ba510f22cb5f502ac";
};
haskellScriptD = import git {};
in derivation {
name = "a"; # giving any name because I won't actually build it.
builder = "a"; # Same here
system = builtins.currentSystem;
script = haskellScriptD;
}
and this is what I run in order to run the executable in the derivative's bin/ subdirectory. You can see I have to append the path before the executable because I don't know how to configure PATH.
nix-shell a.nix --run "\$script/bin/json-generator"
What I want to run is
nix-shell a.nix --run json-generator
How can I do it?

Answer 1

Thanks to #DavidGrayson, I solved it by using stdenv.mkDerivative instead of derivative and including the derivative into buildInputs.
This is the working script
with import <nixpkgs> {};
let git = builtins.fetchGit {
url = https://github.com/ingun37/haskell-script.git;
rev = "e96ac79d6bf338c98697bb4ba510f22cb5f502ac";
};
haskellScriptD = import git {};
in stdenv.mkDerivation {
name = "a";
buildInputs = [ haskellScriptD ];
}
Now I can run
nix-shell a.nix --run json-generator