Creating a random fractional number in F#

Question

I was trying to create a random fractional number as follows:

type Fraction=
  {
  num: int
  den: int
  }

let makeRandomFraction i =
  let n= fun i -> System.Random(i).Next(-50, 51)
  let d= fun i -> System.Random(i*3).Next(-50, 51)
  {num=n; den=d}

but I am getting an error:

error FS0001: This expression was expected to have type
    'int'
but here has type
    'int -> int'

Could you please explain the error and the correct way of doing the required.

Answer 1

The error is saying that you're passing a function (of type int -> int ) where an int is expected. You can see this more clearly in something like:

let add a b = a + b
let inc x = x + 1

inc add
//  ^^^
// This expression was expected to have type 'int' but has type 'int -> int -> int'

The solution here is just to take out the fun i -> parts. That's the (other) syntax for lambdas, but since your i variable is already in scope there's no need to create a function around it.

Answer 2

Out of curiosity, do you have any requirements on what range and distribution of fractions do you want your function to generate? The way the code is currently written - by composing two random numbers in a range -50 .. 50, you will get a distribution with most numbers being close to zero.

Here is a simple histogram built using the XPlot F# library:

open XPlot.GoogleCharts

type Fraction=
  { num: int
    den: int }

let makeRandomFraction i =
  let n = System.Random(i).Next(-50, 51)
  let d = System.Random(i*3).Next(-50, 51)
  {num=n; den=d}

[ for i in 0 .. 100000 -> let f = makeRandomFraction i in float f.num / float f.den ]
|> Seq.filter (System.Double.IsInfinity >> not)
|> Seq.countBy (fun f -> int f)
|> Chart.Column