mkFlake is a utility function that eliminates boilerplate when creating Nix flakes that need to support multiple systems. It’s similar to flake-utils.lib.eachDefaultSystem but with more flexibility.Overview Instead of manually mapping over systems and merging attribute sets, mkFlake handles this automatically:
# Before: Manual system mapping outputs = { self , nixpkgs }: let systems = [ "x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin" ]; in builtins . foldl' ( acc : system : acc // { packages . ${ system } = # ... apps . ${ system } = # ... }) {} systems ; # After: Using mkFlake outputs = { self , nixpkgs , nur }: nur . lib . mkFlake { } ( system : let pkgs = nixpkgs . legacyPackages . ${ system } ; in { packages . hello = pkgs . hello ; apps . greet = { /* ... */ }; } );
Function Signature mkFlake : : { systems :: [String] } -> ( String -> AttrSet ) -> AttrSet
List of systems to generate outputs for. Defaults to the four most common platforms.
Function that takes a system string and returns an attribute set of flake outputs.
Return Value Returns an attribute set with outputs structured as <attr>.<system>.value, except for ignored attributes which are merged directly.
Ignored Attributes The following attributes are not nested under <system> but merged directly:
libsoverlaysnixosModulesnixosConfigurationshydraJobstemplates
This allows you to define system-independent outputs:
nur . lib . mkFlake { } ( system : { packages . myapp = /* system-specific */ ; overlays . default = /* shared across systems */ ; }) ; # Result: # { # packages.x86_64-linux.myapp = ...; # packages.aarch64-linux.myapp = ...; # overlays.default = ...; # NOT nested! # }
Examples Basic Multi-System Flake { inputs = { nixpkgs . url = "github:nixos/nixpkgs/nixos-unstable" ; nur . url = "github:your-org/nur-nix" ; }; outputs = { self , nixpkgs , nur }: nur . lib . mkFlake { } ( system : let pkgs = nixpkgs . legacyPackages . ${ system } ; in { packages . default = pkgs . stdenv . mkDerivation { name = "my-package" ; src = ./src ; }; devShells . default = pkgs . mkShell { packages = with pkgs ; [ git vim ]; }; } ); }
Custom System List If you only need to support Linux systems:
nur . lib . mkFlake { systems = [ "x86_64-linux" "aarch64-linux" ]; } ( system : let pkgs = nixpkgs . legacyPackages . ${ system } ; in { packages . server = pkgs . buildGoModule { # Linux-only server }; } ) ;
Combining with Other Libraries Use mkFlake with mkApps and mkChecks:
{ outputs = { self , nixpkgs , nur }: nur . lib . mkFlake { } ( system : let pkgs = nixpkgs . legacyPackages . ${ system } ; lib = nur . lib . ${ system } ; in { packages . default = /* ... */ ; apps = lib . mkApps { deploy = { script = ''kubectl apply -f ./k8s'' ; deps = [ pkgs . kubectl ]; }; }; checks = lib . mkChecks { format = { script = ''treefmt --check'' ; deps = [ pkgs . treefmt ]; }; }; } ); }
With Overlays and Modules nur . lib . mkFlake { } ( system : let pkgs = nixpkgs . legacyPackages . ${ system } ; in { # System-specific outputs packages . default = pkgs . hello ; # Shared outputs (not nested under system) overlays . default = final : prev : { myPackage = /* ... */ ; }; nixosModules . default = { services . myservice . enable = /* ... */ ; }; } ) ;
Implementation Details Impure System Detection If you run Nix with --impure, mkFlake automatically adds builtins.currentSystem to the systems list if it’s not already included:
if ! builtins ? currentSystem || builtins . elem builtins . currentSystem systems then systems else systems ++ [ builtins . currentSystem ]
This ensures nix build --impure works on any system, even if not in the default list.
Source Location Implemented in: libs/mkFlake/default.nix:1
Comparison with Alternatives Feature mkFlake flake-utils Manual Auto system mapping ✓ ✓ ✗ Custom system list ✓ ✗ ✓ Impure system support ✓ ✗ ✗ Ignored attrs ✓ ✗ ✓ Dependencies None flake-utils None
mkApps Convert script definitions into flake apps
mkChecks Transform check definitions into flake checks
