01 - MetadataResult.fs

namespace Sample

type MetadataResult<'value> = 
    {
        Value: 'value
        Metadata: (string * string) list
    }

module MetadataResult = 
    let init (value: 'a) =
        { Value = value; Metadata = [] }

    let bind (binder: 'a -> 'b) (result: MetadataResult<'a>) = 
        { Value = binder result.Value
        ; Metadata = result.Metadata }

    let logMetadata key value result = 
        { result with 
            Metadata = result.Metadata @ [ key, value ] }

    let logTimestamp result = 
        result 
        |> logMetadata "timestamp" (System.DateTime.Now.ToShortTimeString())

    let print result = 
        let metadata = result.Metadata
        printfn $"Result: {result.Value}"
        printfn "Metadata:"
        for key, value in result.Metadata do
            printfn $"- {key}: {value}"

02 - App.fs

module Sample.App = 

    let createOne () =
        1
        |> MetadataResult.init 
        |> MetadataResult.logMetadata "action" "Started with 1"
        |> MetadataResult.logTimestamp

    let addOne numberResult = 
        numberResult
        |> MetadataResult.bind (fun number -> number + 1)
        |> MetadataResult.logMetadata "action" "+ 1"
        |> MetadataResult.logTimestamp

    let multiplyByTwo numberResult =
        numberResult 
        |> MetadataResult.bind (fun number -> number * 2)
        |> MetadataResult.logMetadata "action" "* 2"
        |> MetadataResult.logTimestamp

    let toString numberResult = 
        numberResult
        |> MetadataResult.bind string
        |> MetadataResult.logMetadata "action" "stringified"
        |> MetadataResult.logTimestamp

    let run () = 
        createOne ()
        |> addOne
        |> multiplyByTwo
        |> toString
        |> MetadataResult.print
        
    

03 - Program.fs

module Sample.Program

let [<EntryPoint>] main _ = 
  App.run ()


Result: 4
Metadata:
- 'action': Started with 1
- 'timestamp': 10:10 PM
- 'action': + 1
- 'timestamp': 10:10 PM
- 'action': * 2
- 'timestamp': 10:10 PM
- 'action': stringified
- 'timestamp': 10:10 PM