f# - Shadowing and Nested function

Question

I want to understand how the mechanism of Shadowing and Nested function work. For example:

let func y =
    let dup y = y + y
    let z = dup y
    let dup y =
        let dup z =
            let y = y * z
            y
        let z = y
        y
    dup z + z;;

val func : int -> int

> func 3;;
val it : int = 12

Can someone explain what happen here?

Answer 1

Your code is equivalent to the following, where I've simply numbered instances of your names to help you visualize how shadowing is occurring.

let func y0 = 
  let dup0 y1 = y1 + y1
  let z0 = dup0 y0
  let dup1 y2 = 
    let dup2 z1 = 
      let y3 = y2 * z1 
      y3
    let z2 = y2 
    y2 
  dup1 z0 + z0

This can be further simplified, of course. Since dup2 and z2 are never used, dup1 is equivalent to let dup1 y2 = y2, and the whole function is equivalent to

let func y0 =
  let dup0 y1 = y1 + y1
  let z0 = dup0 y0
  dup1 z0 + z0

Which is equivalent to

let func y0 =
  let z0 = y0 + y0
  z0 + z0

by substitution. This is the same as

let func y0 = 4 * y0

Does this help?