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Cross-platform

Cross-platform patterns

Configure the shell based on the current machine

Some packages are available only on certain processor architectures or operating systems. A number of helper functions exist in pkgs.stdenv to help you dynamically configure the shell based on the current machine.

A few of the most commonly used functions are:

  • stdenv.isLinux to target machines running Linux

  • stdenv.isDarwin to target machines running macOS

  • stdenv.isAarch64 to target ARM64 processors

  • stdenv.isx86_64 to target X86_64 processors
devenv.nix
{ pkgs, lib, ... }: {
  packages = [
    pkgs.ncdu
  ] ++ lib.optionals pkgs.stdenv.isLinux [
    pkgs.inotify-tools
  ] ++ lib.optionals pkgs.stdenv.isDarwin [
    pkgs.libiconv
  ];

  services.postgres = {
    enable = true;
    settings = {
      log_connections = true;
    } // lib.optionalAttrs pkgs.stdenv.isLinux {
      # Additional settings for Linux systems
    };
  };
}

Advanced conditional configuration with mkIf and mkMerge

For more complex cross-platform configurations, it may be tempting to use // and optionalAttrs in the top-level configuration. This approach will cause Nix to fail with the dreaded infinite recursion error:

devenv.nix
# ❌ This will fail with "error: infinite recursion encountered"
{ pkgs, lib, ... }:

{
  packages = [ pkgs.git ];
} // lib.optionalAttrs pkgs.stdenv.isLinux {
  packages = [ pkgs.ncdu ];
  env.SOME_VAR = "linux-only";
}

error: infinite recursion encountered

The reason this doesn't work is that Nix needs to evaluate the config to figure out the value of conditions like pkgs.stdenv.isLinux. Despite Nix being a lazy language, it needs to be able to strictly evaluate the spine of the top-level attrset—essentially, its keys. This can't happen when the structure itself depends on one of its values.

The solution is to use the module-specific helpers lib.mkIf and lib.mkMerge. mkIf pushes the conditional into the values of the attrset, allowing evaluation to proceed. This function adds extra metadata to the attrset, which is why you then merge multiple conditional blocks with mkMerge.

Use this pattern when you need to conditionally define entire configuration sections, rather than just adding packages or values within existing sections.

devenv.nix
{ pkgs, lib, ... }:

lib.mkMerge [
  {
    # Common packages
    packages = [ pkgs.git ];
  }
  (lib.mkIf pkgs.stdenv.isLinux {
    # Additional Linux packages
    packages = [ pkgs.ncdu ];
    env.SOME_VAR = "linux-only";
  })
]