How can I determine if a list of discriminated union types are of the same case?

Question

Suppose I have a DU like so:

type DU = Number of int | Word of string

And suppose I create a list of them:

[Number(1); Word("abc"); Number(2)]

How can I write a function that would return true for a list of DUs where all the elements are the same case. For the above list it should return false.

Answer 1

The general approach I'd use here would be to map the union values into tags identifying the cases, and then check if the resulting set of tags has at most one element.

let allTheSameCase (tagger: 'a -> int) (coll: #seq<'a>) = 
    let cases = 
        coll
        |> Seq.map tagger
        |> Set.ofSeq
    Set.count cases <= 1 

For the tagger function, you can assign the tags by hand:

 allTheSameCase (function Number _ -> 0 | Word _ -> 1) lst

or use reflection (note that you might need to set binding flags as necessary):

 open Microsoft.FSharp.Reflection

 let reflectionTagger (case: obj) = 
    let typ = case.GetType()
    if FSharpType.IsUnion(typ)
        then 
            let info, _ = FSharpValue.GetUnionFields(case, typ) 
            info.Tag
        else -1 // or fail, depending what makes sense in the context.