So, it's tricky when `Eff` is on the left and right of the function arrow like this. Here are couple of options I've found useful before:

1. If you want to make sure both effect types are the same, but not track the interval effect:

```
foreign import interval :: forall eff r. Number -> Eff eff r -> Eff eff {}
```

If you want to make the interval effect visible:

```
foreign import data Interval :: # ! -> !

foreign import interval :: forall eff eff1 r. Number -> Eff eff r -> Eff (interval :: Interval eff | eff1) {}
```