FsUnit“应等于"在“某些[]"上失败 [英] FsUnit `should equal` fails on `Some []`

问题描述

当我使用NUnit 2.6.3运行此FsUnit测试时，

When I run this FsUnit test with NUnit 2.6.3,

let f xs = Some (List.map ((+) 2) xs)

[<Test>]
let test() =
  f []
  |> should equal (Some [])

我得到:

Result Message: 
Expected: <Some([])>
  But was:  <Some([])>
Result StackTrace:  
at FsUnit.TopLevelOperators.should[a,a](FSharpFunc`2 f, a x, Object y)

即使消息中的期望值"和实际值"相同，测试也会失败.发生什么事了?

The test fails even though the Expected and Actual in the message are the same. What happened?

推荐答案

原因是FsUnit在后台使用了无类型机制，因此类型检查器将Expected推断为object(请参见堆栈跟踪).

The reason is that FsUnit uses untyped mechanism under the hood so Expected is inferred as object by the type checker (see the Object y part in the stacktrace).

一种解决方法是为通用值添加类型注释，即

A workaround is to add type annotation for generic values i.e.

[<Test>]
let test() =
  f []
  |> should equal (Some ([]: int list))

几个人被这种东西咬了. 类型提供程序中的奇怪的无"行为.

Several people have been bitten by this e.g. Weird None behaviour in type providers.

流利的断言之美对我来说毫无意义，一旦它们不再是类型安全的.我建议创建一个类型安全替代:

Beauty of fluent assertions is pointless to me once they're no longer type-safe. I suggest to create a type-safe alternative:

let shouldEqual (x: 'a) (y: 'a) = 
    Assert.AreEqual(x, y, sprintf "Expected: %A\nActual: %A" x y)

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