Implement divide money expressions

Funcky.Money.SourceGenerator/Iso4217RecordGenerator.cs

@@ -158,18 +158,15 @@ private static Option<string> AlphabeticCurrencyName(XmlNode node)
 
     private static Option<int> NumericCurrencyCode(XmlNode node)
         => GetInnerText(node, NumericCurrencyCodeNode)
-            .AndThen(ToInt);
+            .SelectMany(ParseExtensions.ParseInt32OrNone);
 
     private static int MinorUnit(XmlNode node)
         => GetInnerText(node, MinorUnitNode)
-            .AndThen(ToInt)
+            .SelectMany(ParseExtensions.ParseInt32OrNone)
             .GetOrElse(0);
 
     private static Option<string> GetInnerText(XmlNode node, string nodeName)
         => Option
             .FromNullable(node.SelectSingleNode(nodeName))
             .AndThen(n => n.InnerText);
-
-    private static Option<int> ToInt(string s)
-        => s.ParseInt32OrNone();
 }

Funcky.Money.Test/MoneyTest.cs

@@ -1,10 +1,6 @@
-using System;
 using System.Collections;
-using System.Collections.Generic;
-using System.Linq;
 using FsCheck;
 using FsCheck.Xunit;
-using Funcky.Extensions;
 using Xunit;
 
 namespace Funcky.Test;
@@ -38,14 +34,15 @@ public Property TheSumOfTwoMoneysIsCommutative(Money money1, Money money2)
     }
 
     [Fact]
-    public void WeCanBuildTheSumOfTwoMoneysWithDifferentCurrenciesButOnEvaluationYouNeedAnEvaluationContext()
+    public void WeCanBuildTheSumOfTwoMoneysWithDifferentCurrenciesButOnEvaluationYouNeedAnEvaluationContextWithDefinedExchangeRates()
     {
         var fiveFrancs = new Money(5, Currency.CHF);
         var tenDollars = new Money(10, Currency.USD);
 
         var sum = fiveFrancs.Add(tenDollars);
 
         Assert.Throws<MissingEvaluationContextException>(() => sum.Evaluate());
+        Assert.Throws<MissingExchangeRateException>(() => sum.Evaluate(SwissRounding));
     }
 
     [Property]
@@ -302,27 +299,15 @@ public void WeCanDelegateTheExchangeRatesToABank()
 
         var sum = (fiveFrancs + tenDollars + fiveEuros) * 1.5m;
 
-        var context = MoneyEvaluationContext
-            .Builder
-            .Default
-            .WithTargetCurrency(Currency.CHF)
-            .WithBank(OneToOneBank.Instance)
-            .Build();
-
-        Assert.Equal(30m, sum.Evaluate(context).Amount);
+        Assert.Equal(30m, sum.Evaluate(OneToOneContext(Currency.CHF)).Amount);
     }
 
     [Fact]
     public void EvaluationOnZeroMoniesWorks()
     {
         var sum = (Money.Zero + Money.Zero) * 1.5m;
 
-        var context = MoneyEvaluationContext
-            .Builder
-            .Default
-            .WithTargetCurrency(Currency.JPY)
-            .WithBank(OneToOneBank.Instance)
-            .Build();
+        var context = OneToOneContext(Currency.JPY);
 
         Assert.True(Money.Zero.Evaluate(context).IsZero);
         Assert.True(sum.Evaluate(context).IsZero);
@@ -472,6 +457,32 @@ public void RoundingStrategiesMustBeInitializedWithAValidPrecision()
         Assert.Throws<InvalidPrecisionException>(() => _ = RoundingStrategy.RoundWithAwayFromZero(0.0m));
     }
 
+    [Fact]
+    public void WeCanCalculateADimensionlessFactorByDividingAMoneyByAnother()
+    {
+        Assert.Equal(2.5m, Money.CHF(5) / Money.CHF(2));
+        Assert.Equal(0.75m, Money.USD(3).Divide(Money.USD(4)));
+    }
+
+    [Fact]
+    public void DividingTwoMoneysOnlyWorksIfTheyAreOfTheSameCurrency()
+    {
+        Assert.ThrowsAny<MissingExchangeRateException>(() => Money.CHF(5) / Money.USD(2));
+    }
+
+    [Fact]
+    public void DividingTwoMoneysWithDifferentCurrenciesNeedAnEvaluationContext()
+    {
+        Assert.Equal(0.8m, Money.CHF(4).Divide(Money.USD(5), OneToOneContext(Currency.USD)));
+    }
+
+    [Fact]
+    public void DividingTwoMoneysOnlyWorksIfTheDivisorIsNonZero()
+    {
+        Assert.Throws<DivideByZeroException>(() => Money.CHF(5) / Money.Zero);
+        Assert.Throws<DivideByZeroException>(() => Money.USD(3).Divide(Money.USD(0)));
+    }
+
     private static List<decimal> Distributed(SwissMoney someMoney, int numberOfParts)
         => someMoney
             .Get
@@ -500,4 +511,12 @@ private static IMoneyExpression ComplexExpression()
         return v3.Add(v2.Multiply(1.5m).Add(v1)).Add(v2.Multiply(2).Add(v1))
             .Add(v3.Add(v2.Divide(2).Add(v1).Subtract(v2)).Add(v2.Add(v1)));
     }
+
+    private static MoneyEvaluationContext OneToOneContext(Currency targetCurrency)
+        => MoneyEvaluationContext
+            .Builder
+            .Default
+            .WithTargetCurrency(targetCurrency)
+            .WithBank(OneToOneBank.Instance)
+            .Build();
 }

Funcky.Money.Test/TemporaryCultureSwitch.cs

@@ -1,4 +1,3 @@
-using System;
 using System.Globalization;
 
 namespace Funcky.Test;

Funcky.Money/Bank/DefaultBank.cs

@@ -22,7 +22,7 @@ private DefaultBank()
     public decimal ExchangeRate(Currency source, Currency target)
         => ExchangeRates
             .GetValueOrNone(key: (source, target))
-            .GetOrElse(() => throw new NotSupportedException($"No exchange rate for {source} => {target}"));
+            .GetOrElse(() => throw new MissingExchangeRateException($"No exchange rate for {source.AlphabeticCurrencyCode} => {target.AlphabeticCurrencyCode}"));
 
     internal DefaultBank AddExchangeRate(Currency source, Currency target, decimal sellRate)
         => new(ExchangeRates.Add((source, target), sellRate));

Funcky.Money/Exceptions/MissingExchangeRateException.cs

@@ -0,0 +1,26 @@
+using System.Runtime.Serialization;
+
+namespace Funcky;
+
+public class MissingExchangeRateException : Exception
+{
+    public MissingExchangeRateException()
+        : base("if you calculate with more than one currency, you have to define the exchange rate and target in the evaluation context.")
+    {
+    }
+
+    public MissingExchangeRateException(string? message)
+        : base(message)
+    {
+    }
+
+    public MissingExchangeRateException(string? message, Exception? innerException)
+        : base(message, innerException)
+    {
+    }
+
+    protected MissingExchangeRateException(SerializationInfo info, StreamingContext context)
+        : base(info, context)
+    {
+    }
+}

Funcky.Money/ExpressionNodes/IMoneyExpression.cs

@@ -12,6 +12,9 @@ public interface IMoneyExpression
     public static IMoneyExpression operator /(IMoneyExpression dividend, decimal divisor)
         => dividend.Divide(divisor);
 
+    public static decimal operator /(IMoneyExpression dividend, IMoneyExpression divisor)
+        => dividend.Divide(divisor);
+
     public static IMoneyExpression operator +(IMoneyExpression augend, IMoneyExpression addend)
         => augend.Add(addend);
 

Funcky.Money/Extensions/MoneyDivisionExtension.cs

@@ -1,7 +1,19 @@
+using Funcky.Monads;
+
 namespace Funcky;
 
 public static class MoneyDivisionExtension
 {
     public static IMoneyExpression Divide(this IMoneyExpression dividend, decimal divisor)
         => new MoneyProduct(dividend, 1.0m / divisor);
+
+    public static decimal Divide(this IMoneyExpression dividend, IMoneyExpression divisor, Option<MoneyEvaluationContext> context = default)
+        => Divide(
+            dividend.Evaluate(context),
+            divisor.Evaluate(context));
+
+    private static decimal Divide(Money dividend, Money divisor)
+        => dividend.Currency == divisor.Currency
+            ? dividend.Amount / divisor.Amount
+            : throw new MissingExchangeRateException();
 }

Funcky.Money/Funcky.Money.csproj

@@ -6,7 +6,7 @@
         <Description>Funcky.Money is based on Kent Beck's TDD exercise but with more features.</Description>
         <PackageTags>Functional Money</PackageTags>
         <IsPackable>true</IsPackable>
-        <Version>1.1.0</Version>
+        <Version>1.2.0</Version>
     </PropertyGroup>
     <ItemGroup>
         <InternalsVisibleTo Include="$(AssemblyName).Test" />

Funcky.Money/Money.cs

@@ -60,6 +60,9 @@ public bool IsZero
     public static IMoneyExpression operator /(Money dividend, decimal divisor)
         => dividend.Divide(divisor);
 
+    public static decimal operator /(Money dividend, IMoneyExpression divisor)
+        => dividend.Divide(divisor);
+
     private static Currency SelectCurrency(Option<Currency> currency)
         => currency.GetOrElse(CurrencyCulture.CurrentCurrency);
 

Funcky.Money/MoneyEvaluationContext.cs

@@ -1,4 +1,5 @@
 using Funcky.Monads;
+using static Funcky.Functional;
 
 namespace Funcky;
 
@@ -8,8 +9,7 @@ private MoneyEvaluationContext(Currency targetCurrency, Option<decimal> distribu
     {
         TargetCurrency = targetCurrency;
         DistributionUnit = distributionUnit;
-        RoundingStrategy = roundingStrategy
-            .GetOrElse(Funcky.RoundingStrategy.Default(distributionUnit.GetOrElse(Power.OfATenth(TargetCurrency.MinorUnitDigits))));
+        RoundingStrategy = roundingStrategy.GetOrElse(Funcky.RoundingStrategy.Default(distributionUnit.GetOrElse(Power.OfATenth(TargetCurrency.MinorUnitDigits))));
         Bank = bank;
     }
 
@@ -60,14 +60,9 @@ private Builder(Option<Currency> currency, Option<decimal> distributionUnit, Opt
         }
 
         public MoneyEvaluationContext Build()
-        {
-            if (CompatibleRounding().Match(none: false, some: Negate))
-            {
-                throw new IncompatibleRoundingException($"The roundingStrategy {_roundingStrategy} is incompatible with the smallest possible distribution unit {_distributionUnit}.");
-            }
-
-            return CreateContext();
-        }
+            => CompatibleRounding().Match(none: false, some: Not<bool>(Identity))
+                ? throw new IncompatibleRoundingException($"The roundingStrategy {_roundingStrategy} is incompatible with the smallest possible distribution unit {_distributionUnit}.")
+                : CreateContext();
 
         public Builder WithTargetCurrency(Currency currency)
             => With(targetCurrency: currency);
@@ -104,13 +99,9 @@ from unit in _distributionUnit
 
         private MoneyEvaluationContext CreateContext()
             => new(
-                _targetCurrency.GetOrElse(()
-                    => throw new InvalidMoneyEvaluationContextBuilderException("Money evaluation context has no target currency set.")),
+                _targetCurrency.GetOrElse(() => throw new InvalidMoneyEvaluationContextBuilderException("Money evaluation context has no target currency set.")),
                 _distributionUnit,
                 _roundingStrategy,
                 _bank);
-
-        private bool Negate(bool c)
-            => !c;
     }
 }

README.md

@@ -112,13 +112,15 @@ These is the evolving list of TDD requirements which led to the implementation.
 * [x] Money distribution has a precision member, use that instead of the contrived Precision on Rounding.
 * [x] Add unary and binary minus and the division operator.
 * [x] The context has a smallest distribution unit.
+* [x] We can calculate a dimensionless factor by dividing two money objects.
 
 ### Decisions
 
 * We construct `Money` objects only from `decimal` and `int`. The decision how to handle external rounding problems should be done before construction of a `Money` object.
 * We keep Add, Multiply,etc because no all supported frameworks allow default implementations on the interface.
 * We prepare a distribution strategy but do not make it chosable at this point.
 * We support the following operators: unary + and -, and the binary operators ==, !=, +, -, * and /.
+* You can divide to different currencies only with the `Divide(IMoneyExpression, IMoneyExpression, Option<MoneyEvaluationContext>)` method where you have to give a MoneyEvaluationContext with the necessary exchange rates.
 
 ### Open Decisions