F# matching with two values

Question

I'm fairly new to F# and I wanted to compare two values with the (match ... with ...) syntax

The problem arises when I attempt to compare two values like this:

let value1 = 19
let isValue1 y =
    match y with
    | value1 -> y + 1
    | _ -> y

I get a warning that the "| _ -> y" portion of the code will never be reached. Why is this?

I know that I can do the following to get the function to work the way I want it to:

let value1 = 19
let isValue1 y =
    match y with
    | _ when y = value1 -> true
    | _ -> false

This works as well

let value1 = 19
let isValue1 y =
    match y with
    | 19 -> true
    | _ -> false

I'm just curious about why I can't do that, and how match actually works.

Answer 1

The value1 within the match statement is defined as a new variable, the value of which is set to y (as a match). The value1 you define just above is ignored, just as if you were declaring a local variable in a C# function with the same name as a class variable. For this reason, the first match condition will match everything, not just the previously defined value of value1, hence the error. Hope that clarifies matters.