Question

This doesn’t work:

let increment(i: int byref) = i <- i + 1
let xxx = ref 0
increment(xxx) // this expression was expected to have type
               // byref<int> but here has type int ref

But this works:

let incrementParam(a: int byref) = a <- a + 1
let mutable b = 30
incrementParam(&b)

as well as this:

type Incrementor =
    static member Increment(i : int byref) =
       i <- i + 1

let fff = ref 10
Incrementor.Increment(fff) 

Answer 1

Because the spec says so. See Type Directed Conversions at Member Invocations (emphasis mine), especially the following:

Note: These type-directed conversions are primarily for interoperability with existing member-based .NET libraries and do not apply at invocations of functions defined in modules or bound locally in expressions.