Combinator to interpret absence of a field as an empty array

The same way we have jfieldOpt which interpret the absence of a field as a None, we could add a combinator that interpret the absence as an empty array.

How can we call that combinator?

• jfieldMany
• jfieldRepeated
• jfieldArray

Or maybe we can upgrade the existing jfieldOpt to handle multiple values.

[gusty]

I was analyzing this and came up to with an easy way to have this functionality, with the existing combinators.

Let’s pretend we have a pair of functions to go from/to an option of list / plain list:

let flatten = function None -> [] | Some x -> x
let expand = function [] -> None | x  -> Some x

Now, we can use them this way:

type Repeat = { Name : string ; Ids : int list } with
    static member JsonObjCodec =
        fun n i -> { Name = n; Ids = flatten i }
        <!> jreq  "name" (fun x -> Some x.Name)
        <*> jopt  "ids"  (fun x -> expand x.Ids)

And we’ll get the desired effect.

This has the additional advantage that we can apply this to any collection, not just lists and arrays.

[vchekan]

“jfieldRepeated” repeated is protobuf terminology and will be confusing in fleece context.