https://stackoverflow.com/questions/44893428/why-is-f-compiler-sometimes-incorrectly-generalizing-functions

SO44893428

namespace SO44893428

module Generic =
    let nextId =
      let mutable i = 0
      let help (key:obj) =
        i <- i + 1
        i
      help

    let a = nextId "a" // returns 1
    let b = nextId "b" // returns 1


module Concrete =
    let nextId : obj -> int =
      let mutable i = 0
      let help (key:obj) =
        i <- i + 1
        i
      help

    let a = nextId "a" // returns 1
    let b = nextId "b" // returns 2