need to contrast Pattern That Has Type Annotation vs Test Type Pattern

[daveyostcom]

These two are very similar. Elucidating the contrast between them would be very helpful.

---

Document Details

⚠ Do not edit this section. It is required for learn.microsoft.com ➟ GitHub issue linking.

• ID: 8a3ed054-03c9-375f-403b-68f0356d5870
• Version Independent ID: 3c167003-13e8-359b-e919-693258e7f48b
• Content: Pattern Matching - F#
• Content Source: docs/fsharp/language-reference/pattern-matching.md
• Product: dotnet-fsharp
• GitHub Login: @cartermp
• Microsoft Alias: dotnetcontent

[BillWagner]

tagging @vzarytovskii for recommendation on the best fix.

[vzarytovskii]

Test type pattern does type tests (think of it like the as or is operator in C#), for example

type A() = class end
type B() = inherit A()
type C() = inherit A()

let m (a: A) =
    match a with
    | :? B -> printfn "It's a B"
    | :? C -> printfn "It's a C"
    | _ -> ()

Will be a more or less equivalent of

public static void m(A a)
{
    B b = a as B;
    if (a is not null && a is B)
    {
        Console.WriteLine("It's a B");
    }
    else if (a is not null && a is C)
    {
        Console.WriteLine("It's a C");
    }
    else
    {
        return;
    }
}

Whereas patterns with type annotations, it's just telling typechecker which type it is (statically, in compilation time, because everything is expression, you can assign a type with any (sub)expression):

let detect1 x =
    match x with
    | 1 -> printfn "Found a 1!"
    | (var1 : int) -> printfn "%d" var1

Will compile into

public static void detect1(int x)
{
    switch (x)
    {
        case 1:
        {
            Console.WriteLine("Found a 1!");
            break;
        }
        default:
        {
            Console.WriteLine("%d", x);
            break;
        }
    }
}

[daveyostcom]

This level of detail is wonderful. I think it would be better if both examples use the same data.