chore: initial commit

lib/shell/default.nix

@@ -0,0 +1,51 @@
+{ core-inputs
+, user-inputs
+, snowfall-lib
+}:
+
+let
+  inherit (core-inputs.nixpkgs.lib) assertMsg foldl;
+
+  user-shells-root = snowfall-lib.fs.get-file "shells";
+in
+{
+  shell = {
+    # Create flake output packages.
+    # Type: Attrs -> Attrs
+    # Usage: create-shells { inherit channels; src = ./my-shells; overrides = { inherit another-shell; default = "my-shell"; }; }
+    #   result: { another-shell = ...; my-shell = ...; default = ...; }
+    create-shells =
+      { channels
+      , src ? user-shells-root
+      , overrides ? { }
+      }:
+      let
+        user-shells = snowfall-lib.fs.get-default-nix-files-recursive src;
+        create-shell-metadata = shell:
+          {
+            name = builtins.unsafeDiscardStringContext (snowfall-lib.path.get-parent-directory shell);
+            drv = channels.nixpkgs.callPackage shell {
+              lib = snowfall-lib.internal.system-lib;
+            };
+          };
+        shells-metadata = builtins.map create-shell-metadata user-shells;
+        merge-shells = shells: metadata:
+          shells // {
+            ${metadata.name} = metadata.drv;
+          };
+        shells-without-default = foldl merge-shells { } shells-metadata;
+        default-shell =
+          if overrides.default or null == null then
+            { }
+          else if builtins.isAttrs overrides.default then
+            { default = overrides.default; }
+          else if shells-without-default.${overrides.default} or null != null then
+            { default = shells-without-default.${overrides.default}; }
+          else
+            { };
+        overrides-without-default = builtins.removeAttrs overrides [ "default" ];
+        shells = shells-without-default // default-shell // overrides-without-default;
+      in
+      shells;
+  };
+}