F#: Not understanding match .. with

Question

I\'m messing around with F# and Fable, and trying to test my understanding. To do so, I tried creating a function to calculate e given a certain number of iterations. What I

Answer 1

When you use a name in a match statement, you're not checking it against the value assigned to that variable the way you think you are. You are instead assigning that name. I.e.,

match someInt with
| n -> printfn "%d" n

will print the value of someInt. It's the equivalent of let n = someInt; printfn "%d" n.

What you wanted to do was use a when clause; inside a when clause, you're not pattern-matching, but doing a "standard" if check. So what you wanted was:

let eCalc n =
      let rec internalECalc ifact sum count =
          match count with
          | cnt when cnt = n -> sum
          | _ -> internalECalc (ifact / (float count)) (sum + ifact) (count+1)

      internalECalc 1.0 0.0 1

Does that make sense, or do you need me to go into more detail?

P.S. In a case like this one where your match function looks like "x when (boolean condition involving x) -> case 1 | _ -> case 2", it's quite a bit more readable to use a simple if expression:

let eCalc n =
      let rec internalECalc ifact sum count =
          if count = n then
              sum
          else
              internalECalc (ifact / (float count)) (sum + ifact) (count+1)

      internalECalc 1.0 0.0 1