Re: Magical F#

​[sending slightly improved version to blog]

open System

open System.Collections.Generic

type Stats = Map<string, obj>

// Lens code based on http://www.fssnip.net/7Pk/title/Polymorphic-lenses by Vesa Karvonen

type Lens<'s,'t,'a,'b> = ('a -> Option<'b>) -> 's -> Option<'t>

module Lens =

let view l s =

let r = ref Unchecked.defaultof<_>

s |> l (fun a -> r := a; None) |> ignore

!r

let over l f =

l (f >> Some) >> function Some t -> t | _ -> failwith "Impossible"

let set l b = over l <| fun _ -> b

let lens get set = fun f s ->

(get s |> f : Option<_>) |> Option.map (fun f -> set f s)

module Props =

let prop<'t> (propName: string) f =

Lens.lens

(fun x -> match Map.tryFind propName x with | Some(:? 't as v:obj) -> (v |> unbox<'t>) | _ -> Unchecked.defaultof<'t>)

(fun v x -> Map.add propName v x)

f

let set prop v stats = stats |> Lens.set prop (box v)

let get prop stats = stats |> Lens.view prop

open Props

let name = prop<string> "Name"

let id = prop<string> "ID"

let iq = prop<int> "IQ"

let me = Map.empty |> set name "Max" |> set id "30777" |> set iq 4

printfn "%s (%s) is %s" (get name me) (get id me) (if get iq me > 100 then "smart" else "not smart")

// prints "Max (30777) is not smart"

// Did you catch that? iq returns an int, not a boxed int, and name statically knows that it's returning a string! Magic!