A VERY simple application scenario of the Maybe Monad in F#

simple_maybe_monad_usage.fs

open FSharpx

/// Type synonyms
type ProductId = string
type Price = float

type Inventory() =
    let inv_ = new System.Collections.Generic.Dictionary<ProductId, Price>()

    member this.Stock id price =
        inv_.Add(id, price)
    
    member this.Price id =
        inv_ |> Dictionary.tryFind id

let inline (|@|) a b = Option.lift2 (+) a b
    
let reporter =
    function
    | Some p  -> printfn "Total price: %g." p
    | None    -> printfn "One or more id not found."

//Initialize a basic inventory and throw inside a bunch of items
let inventory = Inventory()
inventory.Stock "MyWidget" 10.3
inventory.Stock "Gizmos" 4.34
inventory.Stock "Foo1000" 8.12

//Sum prices
inventory.Price("MyWidget") |@| inventory.Price("Gizmos") |> reporter

inventory.Price("MyWidget") |@| inventory.Price("Gizmos") |@| inventory.Price("Foo1000") |> reporter

//A failing computation
inventory.Price("MyWidget") |@| inventory.Price("Gizmos") |@| inventory.Price("DoesNotExist") |> reporter

//A completely automatic procedure
let sumAndReport (inventory : Inventory) ids =
    let basket = Seq.map (fun pid -> inventory.Price(pid)) ids
    Seq.reduce (|@|) basket |> reporter