C# Code:

using UnityEngine;
using System.Collections;

public class CoroutinesExample : MonoBehaviour
{
    void Update() {
        if (Input.GetKeyDown("f")) {
        StartCoroutine("Fade");
    }

    IEnumerator Fade() {
        for (float f = 1f; f >= 0; f -= 0.1f) {
            Color c = renderer.material.color;
            c.a = f; // alpha
            renderer.material.color = c;
            yield return;
        }
    }

-----------------------------------

F# Code:

namespace CoroutinesExample
open UnityEngine

type CoroutinesExample() =
    inherit MonoBehaviour()

    member this.Update() =
        if Input.GetKey("f") then
            this.StartCoroutine("Fade")
            |> ignore

    member this.Fade() =
        seq { for f in 1.f .. -0.1f .. 0.f do
            let mutable c = this.renderer.material.color
            c.a <- f
            this.renderer.material.color <- c
            yield None }