If I want to use Platform.Sub.map, do I have to import it?

Question

There is no mention of this here.

When I tried, it said Module Platform does not expose Sub

Answer 1

Platform.Sub and Platform.Cmd are imported by default as Sub and Cmd, so you need to omit the Platform.-part.

It's also worth noting that Platform, Platform.Sub, and Platform.Cmd are all different modules, that is, if they weren't imported by default and you just imported Platform, you still couldn't access Sub and Cmd with the Platform. prefix. This is more apparent if you look at the source for the core package (specifically Platform.elm and Platform/).