Testing equality with federated union in a list

Question

I defined different types:

type TypeNull() = class end

type MyType1 = {
    a:int;
    b:int
}

type MyType2 = {
    a:string;
    b:int
}

type MyType3 = {
    a:string;
    b:DateTime
}

and another divided alliance that uses them:

type myDU =
    | A of int
    | B of string
    | C of string

type myDU2 =
    | D of MyType1
    | E of MyType2
    | F of TypeNull

I have a function that maps myDU to myDU2:

let applyArray = function
    | A x -> [E({a="1"; b=2})]
    | B x -> [D({a=1; b=2});E({a="1"; b=2});E({a="5"; b=24})]
    | C x -> [D({a=1; b=2});E({a="1"; b=2});F(TypeNull())]

and then two tests to verify equality:

let arrayValueEquals =
    let expected = [D({a=1; b=2});E({a="1"; b=2});E({a="5"; b=24})]
    let actual = applyArray <| B("xxx")
    actual = expected

let arrayValueNullEquals =
    let expected = [D({a=1; b=2});E({a="1"; b=2});F(TypeNull())]
    let actual = applyArray <| C("xxx")
    actual = expected

What in fsi gives:

val applyArray : _arg1:myDU -> myDU2 list
val arrayValueEquals : bool = true
val arrayValueNullEquals : bool = false

My question is this: why is the first test successful and not the second?

here is the full meaning:

// Learn more about F# at http://fsharp.net. See the 'F# Tutorial' project
// for more guidance on F# programming.

#load "Library1.fs"
open test2
open System

type TypeNull() = class end

type MyType1 = {
    a:int;
    b:int
}

type MyType2 = {
    a:string;
    b:int
}

type MyType3 = {
    a:string;
    b:DateTime
}

type myDU =
    | A of int
    | B of string
    | C of string

type myDU2 =
    | D of MyType1
    | E of MyType2
    | F of TypeNull

let applyArray = function
    | A x -> [E({a="1"; b=2})]
    | B x -> [D({a=1; b=2});E({a="1"; b=2});E({a="5"; b=24})]
    | C x -> [D({a=1; b=2});E({a="1"; b=2});F(TypeNull())]

let arrayValueEquals =
    let expected = [D({a=1; b=2});E({a="1"; b=2});E({a="5"; b=24})]
    let actual = applyArray <| B("xxx")
    actual = expected

let arrayValueNullEquals =
    let expected = [D({a=1; b=2});E({a="1"; b=2});F(TypeNull())]
    let actual = applyArray <| C("xxx")
    actual = expected

Answer 1

In F # there is something called Structural Equality .

In short: lists, arrays, and discriminatory unions support equality if their elements support equality. For lists, this will be an element by comparing the elements.

, , , TypeNull , .

:

type TypeNull() = class end
TypeNull() = TypeNull() // false

let actual = TypeNull()
let expected = TypeNull()
actual = expected // false

, , , true, :

type TypeNull() = class end
let a = TypeNull()
let actual = a
let expected = a
actual = expected // true

DU :

type TypeNull = TypeNull
TypeNull = TypeNull // true

let actual = TypeNull
let expected = TypeNull
actual = expected // True