Name of the DU case

Question

How do I make this test pass by filling out magic?

type DU =
  | ACaseName
  | BThereCake

let magic (q: Quotation<_>): string =
  // smallest F# code in here?

open Expecto
let subject = magic <@ ACaseName @>
Expect.equal subject "ACaseName" "Should extract the NAME of the DU case"

Answer 1

In this case, the following will do:

open Microsoft.FSharp.Quotations

let magic (q: Expr<_>): string =
  match q with 
  | Patterns.NewUnionCase(case, args) -> case.Name
  | _ -> failwith "Not a union case"

let subject = magic <@ ACaseName @>

The question is, what do you want to do when the union case has some arguments. For example:

type DU =
  | ACaseName
  | BThereCake of int

If you wanted to extract the name from <@ BThereCake @> and not just from <@ BThereCake(12) @>, then you need to add one more case:

let magic (q: Expr<_>): string =
  match q with 
  | DerivedPatterns.Lambdas(_, Patterns.NewUnionCase(case, args))
  | Patterns.NewUnionCase(case, args) -> case.Name
  | _ -> failwith "Not a union case"

let subject = magic <@ BThereCake @>