Hidden Features of F#

Answer 1

Not really hidden, but as a non-ML person this escaped me for quite a while:

Pattern matching can decompose arbitrarily deep into data structures.

Here's a [incredibly arbitrary] nested tuple example; this works on lists or unions or any combinations of nested values:

let listEven =
  "Manipulating strings can be intriguing using F#".Split ' '
  |> List.ofArray
  |> List.map (fun x -> (x.Length % 2 = 0, x.Contains "i"), x)
  |> List.choose 
     ( function (true, true), s -> Some s 
              | _, "F#"         -> Some "language" 
              | _               -> None )