Helpful functions for working with pairs in F#

Tuple2.fs

module Tuple2

let replicate x = x, x

let curry f x y = f (x, y)

let uncurry f (x, y) = f x y

let swap (x, y) = (y, x)

let mapFst f (x, y) = f x, y

let mapSnd f (x, y) = x, f y

let extendFst f (x,y) = f (x,y), y

let extendSnd f (x,y) = x, f(x,y)

let optionOfFst f (x, y) =
    match f x with
    | Some x' -> Some (x', y)
    | None    -> None

let optionOfSnd f (x, y) =
    match f y with
    | Some y' -> Some (x, y')
    | None    -> None

Can you add an example of where the extend functions are really useful? The rest I've already got lots of places I can think of where I would have used them.

I got the extend functions from Jérémie Chassaing: https://twitter.com/thinkb4coding/status/775427149243224064

This one is also useful if a and b are of the same type:

let map f (a, b) = f a, f b

I typically call that one mapBoth @brianberns