When to add a unit argument to an existing function in F#

Question

If I am already passing other parameters into a function, do I have any reason to pass in a unit argument as well, assuming the function is non-deterministic? Do I need to worry about memoization repeating return values? Are there any aspects of style or idiom to consider in this case?

Answer 1

If you already have a function that takes some arguments, but also happens to have some side-effects and be non-deterministic, then I would not expect the function to take an additional unit parameter. For example, a randomPoint function would just take maxX and maxY but not additional unit:

let rnd = System.Random()

// Normal way of writing this
let randomPoint maxX maxY = 
  rnd.Next(maxX), rnd.Next(maxY)

let pt1 = randomPoint 100 200

// You can do this, but it is not usual style
let randomPoint maxX maxY () = 
  rnd.Next(maxX), rnd.Next(maxY)

let pt2 = randomPoint 100 200 ()

There are some cases where an extra unit parameter may be helpful, but this is only the case when you want to create a function to be called later. For example:

let makePointGenerator maxX maxY = 
  let rnd = System.Random()
  fun () -> rnd.Next(maxX), rnd.Next(maxY)

// This should be used as follows
let gen = makePointGenerator 100 200
let pt1 = gen () 
let pt2 = gen ()

// You can call it like this, but it is not the intention
let pt3 = makePointGenerator 100 200 ()

Something like this may occasionally be useful for memoization - but this is not something that ever happens automatically and so if you find yourself writing code that requires this, you will know.