Create Discriminated Union Case from String

Question

I\'m trying to create DU cases from strings. The only way I can see doing this is by enumerating over the DU cases via Microsoft.FSharp.Reflection.FSharpType.GetUnionC

Answer 1

I found this handy code snippet...

open Microsoft.FSharp.Reflection

let toString (x:'a) = 
    let (case, _ ) = FSharpValue.GetUnionFields(x, typeof<'a>)
    case.Name

let fromString<'a> (s:string) =
    match FSharpType.GetUnionCases typeof<'a> |> Array.filter (fun case -> case.Name = s) with
    |[|case|] -> Some(FSharpValue.MakeUnion(case,[||]) :?> 'a)
    |_ -> None

... which makes it easy to tack on two lines of code to any DU...

type A = X|Y|Z with
    override this.ToString() = FSharpUtils.toString this
    static member fromString s = FSharpUtils.fromString<A> s

Answer 2

I would use pattern matching like this:

type Keyword = 
    | FOO
    | BAR
    | BAZ
    | BLAH


let matchKeyword (word:string) : Keyword option =
    match word with
    | "FOO"  -> Some FOO
    | "BAR"  -> Some BAR
    | "BAZ"  -> Some BAZ
    | "BLAH" -> Some BLAH
    | _      -> None

And maybe auto generate the match statement first time using regex in my editor, but only because you have hundreds of cases. But i am not sure if its a better solution then yours.

Answer 3

As the cases have no value, another option is to use enums:

type Keyword = 
  | FOO   = 0
  | BAR   = 1
  | BAZ   = 2
  | BLAH  = 3

let strings = ["FOO";"BAR"]
let keywords = 
  [for s in strings -> s, Keyword.Parse(typeof<Keyword>, s)]
  |> Map.ofList

Then you can simply use Enum.Parse.