f# - DU 案件名称

Question

我如何通过填写 magic 来通过此测试?

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

在这种情况下，将执行以下操作:

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 @>

问题是，当工会案有一些争论时，你想做什么。例如:

type DU =
  | ACaseName
  | BThereCake of int

如果您想从 <@ BThereCake @> 中提取名称而不仅仅是来自 <@ BThereCake(12) @> ，那么您需要再添加一个案例:

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 @>