Flakes: Difference between revisions

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 nix flake init
 </syntaxHighlight>
+<translate>
+====Common structure====
+The above command will provide a very simple flake file looking like:
+</translate>
+<syntaxHighlight lang=nix>
+{
+  description = "A very basic flake";
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";
+  };
+  outputs = { self, nixpkgs }: {
+    packages.x86_64-linux.hello = nixpkgs.legacyPackages.x86_64-linux.hello;
+    packages.x86_64-linux.default = self.packages.x86_64-linux.hello;
+  };
+}
+</syntaxHighlight>
+<translate>
+As you can see, it forces you to specify a program for each supported architecture. To avoid this, third-parties projects like flake-utils or flake.parts automatically provide code to avoid this boilerplate. Moreover, in some cases, you may want to also provide an architecture and nixpkgs-independent overlay, allowing for instance the users of the flake to install the program in their system while using their own version of nixpkgs (anything inside <code>packages</code> will always pin all libraries to a fixed version, unless you use <code>yourflake.inputs.nixpkgs.follows = "nixpkgs";</code>). To avoid re-defining the program multiple times (once in the overlay, once for each architecture etc), you can instead write you flake as follows:
+</translate>
+<syntaxHighlight lang=nix>
+{
+  description = "My flake description";
+  # To easily generate a derivation per architecture
+  inputs.flake-utils.url = "github:numtide/flake-utils";
+  outputs = { self, nixpkgs, flake-utils }: {
+    # We define first an overlay, i.e. a definition of new packages as recommended in
+    # https://discourse.nixos.org/t/how-to-consume-a-eachdefaultsystem-flake-overlay/19420/9
+    overlays.default = final: prev: {
+      yourprogram = final.callPackage ({stdenv, pkgs, ...}:
+        # You can put here the derivation to build your program, for instance:
+        stdenv.mkDerivation {
+          src = ./.;
+          pname = "yourprogram";
+          version = "unstable";
+          buildInputs = with pkgs; [
+            # libraries needed by your program
+            # ...
+          ];
+          # Write here additional phases, e.g. buildPhase, installPhase etc.
+          # ...
+        }
+      ) {};
+    };
+  } // # We now add the package defined in the above overlay for all architectures
+  (flake-utils.lib.eachDefaultSystem (system:
+    let
+      pkgs = import nixpkgs {
+        system = system;
+        overlays = [ self.overlays.default ];
+      };
+      lib = nixpkgs.lib;
+    in
+      {
+        # Create a new package
+        packages = {
+          yourprogram = pkgs.yourprogram;
+          default = self.packages.${system}.yourprogram; # default program: this way, typing "nix develop" will directly put you in a shell needed to develop the above your program, running "nix build/run" will directly build/run this program etc.
+        };
+      }
+  ))
+  ;
+}
+</syntaxHighlight>
 <translate>