https://github.com/purescript/purescript-effect/blob/master/src/Effect.js#L12

    return function () {
      return f(a())();
    };

where f is a purescript function. PureScript functions are curried, hence they must accept one and only one argument, but here they call it without arguments.

To speed up calling functions and making FFI easier, I think it could be better for each argument of purescript function has a default value () or None.