module-test.nix

{ pkgs, lib, config, ... }: with lib.types;

let

  postgresqlInstanceType = submodule {
    options.connectionString = lib.mkOption {
      type = str;
    };
  };

in {

  options.services = lib.mkOption {
    type = attrsOf (submodule {
      options.process = lib.mkOption {
        type = str;
      };
      options.args = lib.mkOption {
        type = listOf str;
      };
    });
  };

  options.postgresqlInstances = lib.mkOption {
    type = attrsOf postgresqlInstanceType;
  };

  options.postgresql = lib.mkOption {
    type = postgresqlInstanceType;
  };

  config.services = lib.mapAttrs' (name: cfg:
    lib.nameValuePair "postgresql-${name}" {
      process = "postgresql";
      args = [ cfg.connectionString ];
    }
  }) config.postgresql;

  # mkAliasSomething would also be nice here
  config.postgresqlInstances.default = config.postgresql;


  # ^ Module
  # v User
  
  #config.postgresql.instances.foo.connectionString = "bla";
  config.postgresqlInstances.bar.connectionString = "baz";
  config.postgresql.connectionString = "bla";


}

Not sure what formatting you mean?

must have been a GitHub bug: it was showing no indentation whatsoever but it looks good now 👍