F# Lenses

lens.fs

module Optics
open Microsoft.FSharp.Reflection
open System.Reflection

let get<'b> name r : 'b option =
    let fields = r.GetType() 
                |> FSharpType.GetRecordFields 
                |> Array.where(fun x -> x.Name = name) 
                |> Array.map(fun x -> x.GetValue(r)) 

    match fields |> Array.length with
    | 1 -> (fields |> Array.exactlyOne) :?> 'b |> Some
    | _ -> None

let private removeUntilPeriod (n:string) : string =
    if not (n.Contains(".")) then n 
    else n |> Seq.skipWhile(fun c -> c <> '.') |> Seq.skip 1 |> System.String.Concat

let rec set name (value:'a) (r:'b) : 'b =
    let replacementStrategy (x:PropertyInfo) : obj =
        if x.Name = name then value :> obj
        elif not(name.StartsWith(x.Name)) then x.GetValue(r)
        else
            let name' = removeUntilPeriod name
            let r' = x.GetValue(r)
            set name' value r'
    let values = 
        r.GetType() 
        |> FSharpType.GetRecordFields 
        |> Array.map replacementStrategy
    FSharpValue.MakeRecord (r.GetType(), values) :?> 'b

Program.fs

// Learn more about F# at http://fsharp.org

open System

type Record = {x:int; y:int}
type Nested = {number:int; record:Record}
[<EntryPoint>]
let main argv =
    let record = {x=10; y=20}
    printfn "Creating a record with x=%i and y=%i" record.x record.y
    record |> Optics.get "x" |> printfn "x is %A\n"
    
    printfn "Updating record.x from 10 to 200."
    printfn "Before: %A" record
    record |> Optics.set "x" 200 |> printfn "After: %A\n"

    printfn "Creating a nested record"
    let n = {number=50; record=record}
    printfn "%A\n" n

    printfn "Updating record.x from 10 to 200"
    n |> Optics.set "record.x" 200 |> printfn "%A"
    0 // return an integer exit code