{
  description = "User management system.";

  inputs = {
    flake-utils.url = "github:numtide/flake-utils";
  };

  outputs = inputs @ { self, nixpkgs, ... }:
    inputs.flake-utils.lib.eachDefaultSystem (system:
      let pkgs = nixpkgs.legacyPackages.${system};
          myGo = "${pkgs.go}/bin/go";
          myProtoc = "${pkgs.grpc-tools}/bin/protoc";
          myUsersDir = "$MY_USERS_DIR";
          myUsersBuild = pkgs.writeShellScriptBin "users-build" ''
            pushd ${myUsersDir} > /dev/null &&
            ${myProtoc} --proto_path=${myUsersDir} \
                        --go_out=. --go_opt=paths=source_relative \
                        --go-grpc_out=. --go-grpc_opt=paths=source_relative \
                        proto/users.proto &&
            ${myGo} build -o ${myUsersDir}/bin/users-server ${myUsersDir}/cmd/server/main.go &&
            ${myGo} build -o ${myUsersDir}/bin/users-gateway ${myUsersDir}/cmd/gateway/main.go &&
            popd > /dev/null
          '';
          myUsersTest = pkgs.writeShellScriptBin "users-test" ''
            pushd ${myUsersDir} > /dev/null &&
            ${pkgs.parallel}/bin/parallel ::: ${myUsersDir}/bin/users-server ${myUsersDir}/bin/users-gateway
            popd > /dev/null
          '';
      in
        rec {
          devShells.default = pkgs.mkShell {
            buildInputs = with pkgs; [
              go
              gopls
              protoc-gen-go
              protoc-gen-go-grpc

              grpc
              grpcui
              grpcurl
              grpc-tools

              myUsersBuild
              myUsersTest
            ];

            shellHook = ''
              export MY_USERS_DIR="$(pwd)"
              export GO111MODULE=on
              export GOPATH=$XDG_DATA_HOME/go
              export PATH=$GOPATH/bin:$PATH
              export PATH=$MY_USERS_DIR/bin:$PATH
            '';
          };
        }
    );
}