Installed FSharpx.Core via NuGet.
Replaced Result<'TSuccess, 'TFailure> type with Choice<'T1, 'T2> which is the (monadic) type of the 'Either' computation defined in FSharpx.Core.
Composed the handling of large messages using a Computation Expression.
Used an existing Active Pattern from FSharpx.Core.
Added type aliases for Choice1Of2 and Choice2Of2.
Replaced Choice1Of2 and Choice2Of2 with type aliases.