Question

Can somebody give me an example of how to make inserting data into an F# record more flexible?

I often see examples using records like this:

type Employee = {mutable name:string; mutable id:string}

let data = 
    [{name = "Thomas";id = "000"};
     {name = "Johny";id = "001"};
     {name = "Lucky";id = "002"};
     {name = "Don";id = "003"}
    ]

Can’t we start with no data at all and insert the data into the record later?

(What I mean is without declaration of the value of the data like in the example, so for example: the program is running and asking us to insert the data)

Can we doing something like this with record?

Answer 1

If you’re talking about specifying values of a record as they become available, then you need to make fields of the record option so that you can represent the fact that value is missing. I’ll use immutable records, because this is more common in functional style:

type Employee = { Name:option<string>; ID:option<string> }  

Now you can create a record with only ID and add name when the user enters it:

let empty = { Name = None; ID = Some 123 }
let name = // read name from user
let full = { empty with Name = name }

If you’re talking about adding items to a list as they become available, then you have several options. The direct one is to write a recursive function that repeatedly reads record and builds a list:

let rec readData i records = 
  let name = // read name from user 
  if name <> "" then
    // Create new record and add it to our list
    let itm = { Name = name; ID = string i }
    readData (i + 1) (itm::records)
  else 
    // Return records that we collected so far in the right order
    records |> List.rev

Alternatively, you can also use sequence expressions (see for example free Chapter 12 (PDF) of Real-World Functional Programming). If you user interaction involves waiting for events (e.g. mouse click), then you can still use this style, but you’d need to wrap everything in asynchronous workflow and use Async.AwaitEvent.