strp-enumerator.fsx

// here's the machinery

/// an SRTP function call that can call any type with a Count member, regardless of the return type of the Count
let inline count (t: ^t): ^u =
    (^t : (member Count : ^u) (t))

/// an SRTP function call that can call any Indexer on a type, regardless of the input and output of that indexer
let inline item (t: ^t) (x: ^u) : ^v =
    (^t : (member get_Item: ^u -> ^v) (t, x))

module Seq =
  /// a function that can enumerate over any type with a Count member of type int and an Item indexer that takes an int
  let inline ofGenericItem t=
    seq {
        for x in 0 .. (count t - 1) do
            yield item t x
    }

// here's a custom type
type MyThing (length: int) =
    member x.Count = length
    member x.Item with get (c) =
        if c < length then
            c
        else
            failwith "boom"

// here's using it

let m = MyThing(3)
let numbers = Seq.ofGenericItem m