Add a default value for systems

flake.lock

@@ -20,7 +20,23 @@
     },
     "root": {
       "inputs": {
-        "nixpkgs-lib": "nixpkgs-lib"
+        "nixpkgs-lib": "nixpkgs-lib",
+        "systems": "systems"
+      }
+    },
+    "systems": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
       }
     }
   },

flake.nix

@@ -3,6 +3,7 @@
 
   inputs = {
     nixpkgs-lib.url = "github:NixOS/nixpkgs/nixos-unstable?dir=lib";
+    systems.url = "github:nix-systems/default";
   };
 
   outputs = { self, nixpkgs-lib, ... }: {

modules/perSystem.nix

@@ -1,4 +1,4 @@
-{ config, lib, flake-parts-lib, self, ... }:
+{ config, lib, flake-parts-lib, self, inputsByOutPath, ... }:
 let
   inherit (lib)
     genAttrs
@@ -64,8 +64,16 @@ in
         All the system types to enumerate in the flake output subattributes.
 
         In other words, all valid values for `system` in e.g. `packages.<system>.foo`.
+
+        The default value can be changed by switching to different [nix-systems](https://github.com/nix-systems/nix-systems)
+        input. For example, suppose you create a `my-flake`, which includes a `perSystem.packages.my-package` for
+        default systems, the user of `my-flake` should be able to try out `riscv64-linux` by specifying:
+        ```
+        inputs.my-flake.inputs.flake-parts.inputs.systems.url = "github:nix-systems/riscv64-linux"
+        ```
       '';
       type = types.listOf types.str;
+      default = import (inputsByOutPath ../.).systems;
     };
 
     perInput = mkOption {