How to convert a data array to a record using idiomatic F#

Question

I'm trying to create a communication library that interacts with hardware. The protocol is made up of byte arrays with a header (source/destination address, command number, length) and a command specific payload. I'm creating Record Types for each of the commands to make them more user friendly.

Is there a more idiomatic way of converting an array to a record than

let data = [0;1]
type Rec = {
 A : int
 B : int
}
let convert d =
 {
 A = d.[0]
 B = d.[1]
 }

This can become very tedious when the records are much larger.

Answer 1

A few comments:

You record type definition is bogus - there should be no = in there. I assume you want

type Rec = {
 A : int
 B : int
}

You mentioned byte arrays, but your data value is a List. Accessing List items by index is expensive (O(n)) and should be avoided. If you meant to declare it as an array, the syntax is let data = [|0;1|]

But I wonder if records are the right fit here. If your goal is to have a single function that accepts a byte array and returns back various strongly-typed interpretations of that data, then a discriminated union might be best.

Maybe something along these lines:

// various possible command types
type Commands =
 | Command1 of byte * int // maybe payload of Command1 is known to be an int
 | Command2 of byte * string // maybe payload of Command1 is known to be a string
// active pattern for initial data decomposition
let (|Command|) (bytes : byte[]) =
 (bytes.[0], bytes.[1], Array.skip 2 bytes)
let convert (bytes : byte[]) =
 match bytes with
 | Command(addr, 1uy, [| intData |]) ->
 Command1(addr, int intData)
 | Command(addr, 2uy, strData) ->
 Command2(addr, String(Text.Encoding.ASCII.GetChars(strData)))
 | _ ->
 failwith "unknown command type"
// returns Command1(0x10, 42)
convert [| 0x10uy; 0x01uy; 0x2Auy |]
// returns Command2(0x10, "foobar") 
convert [| 0x10uy; 0x02uy; 0x66uy; 0x6Fuy; 0x6Fuy; 0x62uy; 0x61uy; 0x72uy |]

• jecxjo: Exactly what I was looking for. Sorry I was typing on my phone and wasn't paying attention to syntax at all.