title: What is the simplest way to check that an incoming request contains a certain header value
body: <p>I have a simple Suave.io server in the form:</p>
  
  <pre><code>let Ok o = o |&gt; JsonConvert.SerializeObject |&gt; Successful.OK
  let NotOk o = o |&gt; JsonConvert.SerializeObject |&gt; RequestErrors.BAD_REQUEST
  
  type Result&lt;'T&gt; =
  | Success of 'T
  | Failure of string
  
  let DoThing someParam anotherParam =
      let stats = Success(999) // &lt;- business code here
      match stats with
      | Success s -&gt; s |&gt; Ok
      | Failure m -&gt; m |&gt; NotOk
  ...
          let app =
              choose
                  [ GET &gt;=&gt; choose
                      [
                          pathScan "/someroute/%i/%i" (fun (p1, p2) -&gt; 
                                  DoThing p1 p2)
                      ] 
                  ]
          startWebServer config app
          0
  </code></pre>
  
  <p>I would like to check that the request contains a header with a certain name and value and return NotOk when it is absent or incorrect.  What's the simplest way to achieve this?</p>
  
  <p>I'm a newcomer to Suave.io's compositional style.</p>