Is if .. else .. an idiomatic way of writing things in F#?

Question

What would be an F# idiomatic way of writing the following ? Or would you leave this as is ?

let input = 5
let result = 
     if input > 0 && input  < 5 then
         let a = CalculateA(input)
         let b = CalculateB(input)
         (a+b)/2
     else
         CalculateC(input)

Answer 1

For one if ... then ... else ... I'd probably leave it like that, if you had more cases I'd either use pattern match with a when guard:

let result =
    match input with
    | _ when input > 0 && input < 5  -> ...
    | _ -> ...

or you might also want to look at active patterns: http://msdn.microsoft.com/en-us/library/dd233248.aspx

Answer 2

What would be an F# idiomatic way of writing the following ? Or would you leave this as is ?

There's nothing wrong with the way you've written it but here is another alternative (inspired by Huusom):

let input = 5
let result =
  if input>0 && input<5 then [A; B] else [C]
  |> Seq.averageBy (fun f -> f input)