From e7c07009592b1aa6d1c0fbdeaa6d62a95d991803 Mon Sep 17 00:00:00 2001
From: Timothy DeHerrera <tim.deh@pm.me>
Date: Mon, 28 Dec 2020 23:08:11 -0700
Subject: [PATCH] factor out a generic fetch from nixpkgs-compat

Create fetch function which takes a valid flake input, and calls
fetchTarball with the revision and hash information from the flake.lock.

Useful for pulling the exact revision of flake inputs from not flake
expressions, such as those in shell.nix.
---
 compat/fetch.nix   | 17 +++++++++++++++++
 compat/nixpkgs.nix |  5 +++++
 nixpkgs-compat.nix | 13 -------------
 shell.nix          |  2 +-
 4 files changed, 23 insertions(+), 14 deletions(-)
 create mode 100644 compat/fetch.nix
 create mode 100644 compat/nixpkgs.nix
 delete mode 100644 nixpkgs-compat.nix

diff --git a/compat/fetch.nix b/compat/fetch.nix
new file mode 100644
index 00000000..01f0f02f
--- /dev/null
+++ b/compat/fetch.nix
@@ -0,0 +1,17 @@
+let
+  inherit (builtins)
+    fetchTarball
+    fromJSON
+    readFile
+    ;
+  lockfile = fromJSON (readFile ../flake.lock);
+in
+input:
+let
+  locked = lockfile.nodes."${input}".locked;
+  inherit (locked) rev narHash owner repo;
+in
+fetchTarball {
+  url = "https://github.com/${owner}/${repo}/archive/${rev}.tar.gz";
+  sha256 = narHash;
+}
diff --git a/compat/nixpkgs.nix b/compat/nixpkgs.nix
new file mode 100644
index 00000000..3dd503a9
--- /dev/null
+++ b/compat/nixpkgs.nix
@@ -0,0 +1,5 @@
+let
+  fetch = import ./fetch.nix;
+  nixpkgs = fetch "nixos";
+in
+nixpkgs
diff --git a/nixpkgs-compat.nix b/nixpkgs-compat.nix
deleted file mode 100644
index 08b07372..00000000
--- a/nixpkgs-compat.nix
+++ /dev/null
@@ -1,13 +0,0 @@
-let
-  inherit (builtins)
-    fetchTarball
-    fromJSON
-    readFile
-    ;
-  nixos = (fromJSON (readFile ./flake.lock)).nodes.nixos.locked;
-  nixpkgs = fetchTarball {
-    url = "https://github.com/NixOS/nixpkgs/archive/${nixos.rev}.tar.gz";
-    sha256 = nixos.narHash;
-  };
-in
-nixpkgs
diff --git a/shell.nix b/shell.nix
index 5aae2efc..dadb9e7f 100644
--- a/shell.nix
+++ b/shell.nix
@@ -1,5 +1,5 @@
 let
-  nixpkgs' = import ./nixpkgs-compat.nix;
+  nixpkgs = import ./compat/nixpkgs.nix;
 in
 { pkgs ? import nixpkgs { }, nixpkgs ? nixpkgs' }:
 let