I had quite the frustrating hour with Suave trying to combine Webparts. In theory it is very simple:

Function1-that-returns-Webpart >=> Function2-that-returns-Webpart

In practice I kept getting an error “Expecting a type supporting the operator ‘>=>’ but given a function type. You may be missing an argument to a function” when trying to run this (thinned out) code:

let getTemplate : Webpart =
    OK "..." 
    >=> Writers.setMimeType "application/json; charset=utf-8"

let route = choose [ pathScan Path.Assets.templateById getTemplate ]
startWebServer config route

The problem – a missing open Suave.Operators – and without it F# didn’t know what >=> was. I guess it was trying to treat >=> as an inline function, but how it goes from there to that error message presently baffles me.