Data format selection with static abstract member-based encoder

Encodable.fs

type IEncodable<'t> =
    static abstract Encode : 't -> byte[]

/// Selects Base64 encoding for `a`.
type Base64<'a when 'a :> IEncodable<'a>> = Base64 of 'a with
    
    interface IEncodable<Base64<'a>> with
        member this.Encode (Base64 a) =
            let toBase64 = failwith "..."
            toBase64 ('a.Encode a)

// Default encoding
let bytes = string.Encode "Hello"

// Base64 encoding
let byte64 = Base64<_>.Encode (Base64 "Hello") // Hypothetical syntax