FsUnit.Xunit list should contain an x such that f x

Question

I am not sure how to write my test in FsUnit.Xunit.

I want to check that at least one element in my list satisfies a predicate. I know how to write this using should be True, but usually you can get better error messages by using the specialized functions.

Hopefully this code makes it clear what I am trying to achieve:

open Xunit
open FsUnit.Xunit

type Foo = 
  {
    X : int
  }

[<Fact>]
let ``squares`` () =
  let xs =
    [ { X = 1 }; { X = 2 }; { X = 3 } ]
    |> List.map (fun foo -> { X = foo.X * foo.X })

  actual 
  |> should exists (satisfies (fun foo -> foo.X = 9)) // Not real code

Answer 1

I would do this:

xs
    |> Seq.map (fun foo -> foo.X)
    |> should contain 9

Error output is useful:

Expected: Contains 8
Actual:   seq [1; 4; 9]

If you want more context, you can do this instead:

open FsUnit.CustomMatchers

...

xs |> should containf (fun foo -> foo.X = 9)

Error output is:

Expected: Contains <fun:squares@21-1>
Actual:   [{ X = 1 }; { X = 4 }; { X = 9 }]

The only downside of this approach is that the "Expected" message no longer displays the specific value you're looking for.