use generic math for IBank

Funcky.Money.Test/MoneyTest.cs

@@ -517,6 +517,6 @@ private static MoneyEvaluationContext OneToOneContext(Currency targetCurrency)
             .Builder
             .Default
             .WithTargetCurrency(targetCurrency)
-            .WithBank(OneToOneBank.Instance)
+            .WithBank(OneToOneBank<decimal>.Instance)
             .Build();
 }

Funcky.Money.Test/OneToOneBank.cs

@@ -1,10 +1,13 @@
+using System.Numerics;
+
 namespace Funcky.Test;
 
 // Bank which returns a 1:1 exchange rate for every pair of currencies.
-internal sealed class OneToOneBank : IBank
+internal sealed class OneToOneBank<TUnderlyingType> : IBank<TUnderlyingType>
+    where TUnderlyingType : INumberBase<TUnderlyingType>
 {
-    public static readonly IBank Instance = new OneToOneBank();
+    public static readonly IBank<TUnderlyingType> Instance = new OneToOneBank<TUnderlyingType>();
 
-    public decimal ExchangeRate(Currency source, Currency target)
-        => 1m;
+    public TUnderlyingType ExchangeRate(Currency source, Currency target)
+        => TUnderlyingType.One;
 }

Funcky.Money/Bank/DefaultBank.cs

@@ -1,29 +1,31 @@
 using System.Collections.Immutable;
+using System.Numerics;
 using Funcky.Extensions;
 
 namespace Funcky;
 
-internal sealed class DefaultBank : IBank
+internal sealed class DefaultBank<TUnderlyingType> : IBank<TUnderlyingType>
+    where TUnderlyingType : INumberBase<TUnderlyingType>
 {
-    internal static readonly DefaultBank Empty = new();
+    internal static readonly DefaultBank<TUnderlyingType> Empty = new();
 
-    public DefaultBank(ImmutableDictionary<(Currency Source, Currency Target), decimal> exchangeRates)
+    public DefaultBank(ImmutableDictionary<(Currency Source, Currency Target), TUnderlyingType> exchangeRates)
     {
         ExchangeRates = exchangeRates;
     }
 
     private DefaultBank()
     {
-        ExchangeRates = ImmutableDictionary<(Currency Source, Currency Target), decimal>.Empty;
+        ExchangeRates = ImmutableDictionary<(Currency Source, Currency Target), TUnderlyingType>.Empty;
     }
 
-    public ImmutableDictionary<(Currency Source, Currency Target), decimal> ExchangeRates { get; }
+    public ImmutableDictionary<(Currency Source, Currency Target), TUnderlyingType> ExchangeRates { get; }
 
-    public decimal ExchangeRate(Currency source, Currency target)
+    public TUnderlyingType ExchangeRate(Currency source, Currency target)
         => ExchangeRates
             .GetValueOrNone(key: (source, target))
             .GetOrElse(() => throw new MissingExchangeRateException($"No exchange rate for {source.AlphabeticCurrencyCode} => {target.AlphabeticCurrencyCode}."));
 
-    internal DefaultBank AddExchangeRate(Currency source, Currency target, decimal sellRate)
+    internal DefaultBank<TUnderlyingType> AddExchangeRate(Currency source, Currency target, TUnderlyingType sellRate)
         => new(ExchangeRates.Add((source, target), sellRate));
 }

Funcky.Money/Bank/IBank.cs

@@ -1,6 +1,6 @@
 namespace Funcky;
 
-public interface IBank
+public interface IBank<TUnderlyingType>
 {
-    decimal ExchangeRate(Currency source, Currency target);
+    TUnderlyingType ExchangeRate(Currency source, Currency target);
 }

Funcky.Money/MoneyEvaluationContext.cs

@@ -5,7 +5,7 @@ namespace Funcky;
 
 public sealed class MoneyEvaluationContext
 {
-    private MoneyEvaluationContext(Currency targetCurrency, Option<decimal> distributionUnit, Option<IRoundingStrategy<decimal>> roundingStrategy, IBank bank)
+    private MoneyEvaluationContext(Currency targetCurrency, Option<decimal> distributionUnit, Option<IRoundingStrategy<decimal>> roundingStrategy, IBank<decimal> bank)
     {
         TargetCurrency = targetCurrency;
         DistributionUnit = distributionUnit;
@@ -32,7 +32,7 @@ private MoneyEvaluationContext(Currency targetCurrency, Option<decimal> distribu
     /// <summary>
     /// Source for exchange rates.
     /// </summary>
-    public IBank Bank { get; }
+    public IBank<decimal> Bank { get; }
 
     public sealed class Builder
     {
@@ -41,17 +41,17 @@ public sealed class Builder
         private readonly Option<Currency> _targetCurrency;
         private readonly Option<decimal> _distributionUnit;
         private readonly Option<IRoundingStrategy<decimal>> _roundingStrategy;
-        private readonly IBank _bank;
+        private readonly IBank<decimal> _bank;
 
         private Builder()
         {
             _targetCurrency = default;
             _roundingStrategy = default;
             _distributionUnit = default;
-            _bank = DefaultBank.Empty;
+            _bank = DefaultBank<decimal>.Empty;
         }
 
-        private Builder(Option<Currency> currency, Option<decimal> distributionUnit, Option<IRoundingStrategy<decimal>> roundingStrategy, IBank bank)
+        private Builder(Option<Currency> currency, Option<decimal> distributionUnit, Option<IRoundingStrategy<decimal>> roundingStrategy, IBank<decimal> bank)
         {
             _targetCurrency = currency;
             _distributionUnit = distributionUnit;
@@ -71,17 +71,17 @@ public Builder WithRounding(IRoundingStrategy<decimal> roundingStrategy)
             => With(roundingStrategy: Option.Some(roundingStrategy));
 
         public Builder WithExchangeRate(Currency currency, decimal sellRate)
-            => _bank is DefaultBank bank
+            => _bank is DefaultBank<decimal> bank
                 ? With(bank: bank.AddExchangeRate(currency, GetTargetCurrencyOrException(), sellRate))
                 : throw new InvalidMoneyEvaluationContextBuilderException("You can either use WithExchangeRate or WithBank, but not both.");
 
-        public Builder WithBank(IBank bank)
+        public Builder WithBank(IBank<decimal> bank)
             => With(bank: Option.Some(bank));
 
         public Builder WithSmallestDistributionUnit(decimal distributionUnit)
             => With(distributionUnit: distributionUnit);
 
-        private Builder With(Option<Currency> targetCurrency = default, Option<decimal> distributionUnit = default, Option<IRoundingStrategy<decimal>> roundingStrategy = default, Option<IBank> bank = default)
+        private Builder With(Option<Currency> targetCurrency = default, Option<decimal> distributionUnit = default, Option<IRoundingStrategy<decimal>> roundingStrategy = default, Option<IBank<decimal>> bank = default)
             => new(
                 targetCurrency.OrElse(_targetCurrency),
                 distributionUnit.OrElse(_distributionUnit),