Dynamic Lookup in F#

Question

Can somebody help me with article of Tomas Petricek: http://tomasp.net/blog/fsharp-dynamic-lookup.aspx/#dynfslinks? The problem is that it is severely outdated. I understand that namespaces

open Microsoft.FSharp.Quotations.Typed
open Microsoft.FSharp.Quotations.Raw

are gone. So I removed the openings. But there are still errors. "Typed" is not defined. "RecdGet" is not defined. And I suspect they are not the last. I'm trying to prove to my boss that F# is good to use for database normalization. Dynamic lookup of fields would really helped me to deal with similarly named fields having different prefixes.

There is also post of Tomas on fpish: https://fpish.net/topic/None/57493, which I understand predates the article

Answer 1

Here's a rough equivalent:

open Microsoft.FSharp.Quotations
open Microsoft.FSharp.Quotations.Patterns

type DynamicMember<'t,'u> = Expr<'t -> 'u>

let getValueReader (expr:DynamicMember<'recdT, 'fieldT>) = 
  // Match the quotation representing the symbol
  match expr with
  | Lambda(v, PropertyGet (Some (Var v'), pi, [])) when v = v' ->
      // It represents reading of the F# record field..
      // .. get a function that reads the record field using F# reflection
      let rdr = Reflection.FSharpValue.PreComputeRecordFieldReader pi

      // we're not adding any additional processing, so we just
      // simply add type conversion to the correct types & return it
      ((box >> rdr >> unbox) : 'recdT -> 'fieldT)
  | _ -> 
      // Quotation doesn't represent symbol - this is an error
      failwith "Invalid expression - not reading record field!"

type SampleRec = { Str : string; Num : int }

let readStrField = getValueReader <@ fun (r : SampleRec) -> r.Str @>
let readNumField = getValueReader <@ fun (r : SampleRec) -> r.Num @>   

let rc = { Str = "Hello world!"; Num = 42 }
let s, n = readStrField rc, readNumField rc

printfn "Extracted: %s, %d" s n