3 ways of organising F# functions related to a type

1-plain-function.fs

module VendingMachine

type Coin = Quarter | Dime | Nickel

let valueOf coin =
  match coin with
  | Quarter -> 0.25m
  | Dime    -> 0.10m
  | Nickel  -> 0.05m

2-members.fs

/// Easier to discover as you can just type a '.' in VS to see options.
/// Downside - '.' breaks the type inference :(
module VendingMachine

type Coin =
  | Quarter
  | Dime
  | Nickel
  member this.Value =
    match this with
      | Quarter -> 0.25m
      | Dime    -> 0.10m
      | Nickel  -> 0.05m

3-matching-module.fs

/// This is a common pattern for F# libraries to follow - to keep the type simple, 
/// but make it easy to find associated functions, and gives good type inference.
module VendingMachine

type Coin = Quarter | Dime | Nickel

// The attribute lets us use the same name as the type.
[<CompilationRepresentation(CompilationRepresentationFlags.ModuleSuffix)>]
module Coin =
  let value coin =
    match coin with
    | Quarter -> 0.25m
    | Dime    -> 0.10m
    | Nickel  -> 0.05m