RefTest.cs

using System.Linq;
using Xunit;

namespace LanguageExt.Tests;

public class Account
{
    public readonly int Balance;

    Account(int balance) =>
        Balance = balance;

    public Account SetBalance(int value) =>
        new Account(value);

    public Account AddBalance(int value) =>
        new Account(Balance + value);

    public Account Deposit(int value) =>
        new Account(Balance + value);

    public static Ref<Account> New(int balance) =>
        Ref(new Account(balance), Account.Validate);

    public static bool Validate(Account a) =>
        a.Balance >= 0;

    public override string ToString() =>
        Balance.ToString();
}

public static class Transfer
{
    public static Unit Do(Ref<Account> from, Ref<Account> to, int amount) => 
        atomic(() =>
               {
                   from.Value = from.Value.AddBalance(-amount);
                   to.Value   = to.Value.AddBalance(amount);
               });
}


public class RefTest
{
    [Fact]
    public void BankBalanceChangeTest()
    {
        var accountA = Account.New(200);
        var accountB = Account.New(0);

        var stateA   = Option<Account>.None;
        var stateB   = Option<Account>.None;
        var changedA = 0;
        var changedB = 0;
        accountA.Change += v => { stateA = v; changedA++; };
        accountB.Change += v => { stateB = v; changedB++; };

        atomic(() =>
               {
                   accountA.Value = accountA.Value.AddBalance(-50);
                   accountB.Value = accountB.Value.AddBalance(50);
                   accountA.Value = accountA.Value.AddBalance(-5);
                   accountB.Value = accountB.Value.AddBalance(5);

                   Assert.Equal(None, stateA);
                   Assert.Equal(None, stateB);
               });

        Assert.Equal(Some(accountA.Value), stateA);
        Assert.Equal(Some(accountB.Value), stateB);
        Assert.Equal(1, changedA);
        Assert.Equal(1, changedB);
    }

    [Fact]
    public void SimpleBankBalanceTest()
    {
        var accountA = Account.New(200);
        var accountB = Account.New(0);

        Transfer.Do(accountA, accountB, 100);

        var (balanceA, balanceB) = atomic(() => (accountA.Value.Balance, accountB.Value.Balance));

        Assert.True(balanceA == balanceB);
    }

    [Fact]
    public void CommuteTest()
    {
        const int count = 1000;

        static int inc(Ref<int> counter) => 
            atomic(() => commute(counter, static x => x + 1));

        var num = Ref(0);

        var res = Range(0, count).AsParallel()
                                 .Select(_ => inc(num))
                                 .AsIterable() 
                                 .ToSeq()
                                 .Strict();

        Assert.True(num == count);
    }

    [Fact]
    static void DepositCommuteTest()
    {
        var bank = Account.New(0);

        LogDeposit(bank, 100);
        LogDeposit(bank, 50);

        Assert.True(bank.Value.Balance == 150);

        var bank2 = Account.New(0);

        LogDeposit(bank2, 50);
        LogDeposit(bank2, 100);

        Assert.True(bank2.Value.Balance == 150);

    }

    static void LogDeposit(Ref<Account> account, int amount) =>
        atomic(() => commute(account, a => a.Deposit(amount)));
}