Add exercise 3 (Buyers, C#): the pure extract-lambda rep

New in-theme classes: Buyers computes a shopping-list total, PriceList
stands for the pricing service (random surge, flaky test). One loop, one
lambda-able expression — the third example teaches the move with nothing
else in the way. Pristine repo + reference solution (extract lambda,
slide, extract method with Func<IceCream,int> parameter, pure-lambda
test) + storyboard in the established style. Mirrors the kata's C#
setup (net7.0, MSTest); needs a build check on a machine with the SDK.

ideas/split-loop-with-lambda/exercise3/README.md

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+# Exercise 3 — Buyers (Split Loop with Lambda, the pure rep)
+
+A third, minimal example in the same kata theme: the sellers exist
+(`DailyForecasts` — Steve, Julie, Francis), the buyers don't. This adds them:
+`Buyers` computes a buyer's shopping-list total, and `PriceList` stands for an
+external pricing service (it adds random surge pricing, so the test is flaky).
+
+Written in **C#** (MSTest, .NET 7 — same setup as the kata's `csharp/` folder).
+Run with `dotnet test test/BuyersTests.csproj`.
+
+Contents:
+
+- `csharp/` — pristine exercise repo: one loop, **one** lambda-able expression
+  (`priceList.PriceFor(flavour)`), nothing else. No printing, no conditionals,
+  no void calls.
+- `solution/` — end state: extracted static method taking
+  `Func<IceCream, int> priceFor`, a no-arg overload passing the method group,
+  and a deterministic test with a pure lambda (`_ => 2`).
+
+`IceCream.cs` is copied from
+[IceCreamForecasting-Refactoring-Kata](https://github.com/emilybache/IceCreamForecasting-Refactoring-Kata)
+(same license as the kata applies).
+
+## Where this sits in the sequence
+
+| example | lambda | teaches |
+|---|---|---|
+| 1. `longRangeForecast` weather loop (Java) | `LongFunction<Boolean>` | the pattern + the drift trap |
+| 3. `Buyers` shopping list (C#, this one) | `Func<IceCream, int>` | the move in its purest form: one expression, nothing else |
+| demo/next: `printSalesForecasts` → `SalesScorer`, `PriceList` → `Pricing` | role interface | when one function isn't enough (two side-effectful methods to stub) |
+
+Next-time arc: marketing adds surge discounts, `PriceList` grows
+`DiscountFor(...)` — two methods to stub → extract a role interface
+(`IPricing`) instead of two lambdas. See the demo script's closing note.
+
+## Not verified by a build
+
+This exercise was authored without a local .NET SDK (structurally mirrored from
+the kata's `csharp/` project files). Run `dotnet test test/BuyersTests.csproj`
+once on a machine with the SDK before facilitating.

ideas/split-loop-with-lambda/exercise3/csharp/global.json

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+{
+  "sdk": {
+    "version": "7.0.0",
+    "rollForward": "latestMajor",
+    "allowPrerelease": true
+  }
+}

ideas/split-loop-with-lambda/exercise3/csharp/src/Buyer.cs

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+using System;
+using System.Collections.Generic;
+
+namespace IceCreamBuyers
+{
+    public class Buyer
+    {
+        public Buyer(string name, IReadOnlyList<IceCream> favourites)
+        {
+            Name = name;
+            Favourites = favourites;
+        }
+
+        public string Name { get; }
+        public IReadOnlyList<IceCream> Favourites { get; }
+
+        public int ScoopsOf(IceCream flavour)
+        {
+            return 2;
+        }
+    }
+}

ideas/split-loop-with-lambda/exercise3/csharp/src/Buyers.cs

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+using System;
+using System.Collections.Generic;
+
+namespace IceCreamBuyers
+{
+    public class Buyers
+    {
+        private readonly PriceList priceList = new();
+        private readonly List<Buyer> buyers = new()
+        {
+            new("Anke", new List<IceCream> { IceCream.Strawberry, IceCream.Vanilla }),
+            new("Bram", new List<IceCream> { IceCream.Chocolate }),
+            new("Celine", new List<IceCream> { IceCream.Vanilla, IceCream.Chocolate }),
+        };
+
+        public int ShoppingListTotal(string buyerName)
+        {
+            var buyer = ByName(buyerName);
+            var total = 0;
+            foreach (var flavour in buyer.Favourites)
+            {
+                total += buyer.ScoopsOf(flavour) * priceList.PriceFor(flavour);
+            }
+            return total;
+        }
+
+        private Buyer ByName(string name)
+        {
+            return buyers.Find(b => b.Name == name)
+                ?? throw new InvalidOperationException($"Unknown buyer {name}.");
+        }
+    }
+}

ideas/split-loop-with-lambda/exercise3/csharp/src/Buyers.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net7.0</TargetFramework>
+  </PropertyGroup>
+
+</Project>

ideas/split-loop-with-lambda/exercise3/csharp/src/IceCream.cs

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+namespace IceCreamBuyers
+{
+    public enum IceCream
+    {
+        Strawberry,
+        Chocolate,
+        Vanilla
+    }
+}

ideas/split-loop-with-lambda/exercise3/csharp/src/PriceList.cs

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+using System;
+using System.Collections.Generic;
+
+namespace IceCreamBuyers
+{
+    public class PriceList
+    {
+        // placeholder implementation - real version would call the pricing service
+        private static readonly Dictionary<IceCream, int> BasePrices =
+            new()
+            {
+                { IceCream.Strawberry, 3 },
+                { IceCream.Vanilla, 2 },
+                { IceCream.Chocolate, 4 },
+            };
+
+        public int PriceFor(IceCream flavour)
+        {
+            return BasePrices[flavour] + SurgePricing();
+        }
+
+        private int SurgePricing()
+        {
+            return new Random().Next(3);
+        }
+    }
+}

ideas/split-loop-with-lambda/exercise3/csharp/test/BuyersTests.cs

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+using IceCreamBuyers;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+namespace IceCreamBuyers.Tests
+{
+    [TestClass]
+    public class BuyersTests
+    {
+        [TestMethod]
+        public void ShoppingListTotal()
+        {
+            // Anke: 2 flavours x 2 scoops x base prices (3 + 2) = 10.
+            // Red or flaky — why? (PriceList adds random surge pricing;
+            // it stands for the external pricing service.)
+            var buyers = new Buyers();
+            Assert.AreEqual(10, buyers.ShoppingListTotal("Anke"));
+        }
+    }
+}

ideas/split-loop-with-lambda/exercise3/csharp/test/BuyersTests.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net7.0</TargetFramework>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.7.1" />
+    <PackageReference Include="MSTest.TestAdapter" Version="2.1.1" />
+    <PackageReference Include="MSTest.TestFramework" Version="2.1.1" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\src\Buyers.csproj" />
+  </ItemGroup>
+
+</Project>

ideas/split-loop-with-lambda/exercise3/solution/README.md

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+# Reference solution — end state of exercise 3
+
+Derived from [IceCreamForecasting-Refactoring-Kata](https://github.com/emilybache/IceCreamForecasting-Refactoring-Kata)
+(same license as the kata applies to the copied `IceCream.cs`).
+
+Run with `dotnet test test/BuyersTests.csproj` — the test passes deterministically
+without touching the pricing service.
+
+## What changed vs the exercise start
+
+1. The pricing call extracted into a lambda and slid to the top:
+   `Func<IceCream, int> priceFor = flavour => priceList.PriceFor(flavour);`
+   — the invocation in the loop stays put (`priceFor(flavour)`).
+2. The loop extracted to
+   `ShoppingListTotal(Buyer buyer, Func<IceCream, int> priceFor)` — static-free of
+   `PriceList`; all inputs are parameters. (C# note: this one is an instance method
+   because `ByName` uses the `buyers` field; the *extracted loop method itself*
+   touches no fields. If you want it `static`, extract the loop only and keep the
+   lookup in the composed method.)
+3. No-arg `ShoppingListTotal(string)` overload keeps callers working; it passes the
+   method group `priceList.PriceFor` as the "real" pricing function.
+4. The test passes a pure lambda (`_ => 2`) through the overload — deterministic,
+   glanceable: 2 flavours × 2 scoops × 2 = 8.
+
+## Intermediate step (between pristine and this solution)
+
+```csharp
+public int ShoppingListTotal(string buyerName)
+{
+    Func<IceCream, int> priceFor = flavour => priceList.PriceFor(flavour); // ← extracted & slid
+    var buyer = ByName(buyerName);
+    var total = 0;
+    foreach (var flavour in buyer.Favourites)
+    {
+        total += buyer.ScoopsOf(flavour) * priceFor(flavour);              // ← invocation stays
+    }
+    return total;
+}
+```
+
+## Variations (debrief / second round)
+
+- `_ => 5` — everything costs 5: Anke 20, Bram 10, Celine 20.
+- `flavour => flavour == IceCream.Chocolate ? 10 : 1` — chocolate is expensive.
+- Delete `PriceList` entirely: the extracted method no longer knows it exists —
+  a step-4 move is now unblocked (move `ShoppingListTotal(Buyer, Func)` to a
+  class without `PriceList`).
+- Role-interface next step: add `DiscountFor(IceCream)` to `PriceList` (a second
+  side-effectful method) and feel why one lambda is no longer enough — extract
+  `IPricing` instead.

ideas/split-loop-with-lambda/exercise3/solution/global.json

@@ -0,0 +1,7 @@
+{
+  "sdk": {
+    "version": "7.0.0",
+    "rollForward": "latestMajor",
+    "allowPrerelease": true
+  }
+}

ideas/split-loop-with-lambda/exercise3/solution/src/Buyer.cs

@@ -0,0 +1,22 @@
+using System;
+using System.Collections.Generic;
+
+namespace IceCreamBuyers
+{
+    public class Buyer
+    {
+        public Buyer(string name, IReadOnlyList<IceCream> favourites)
+        {
+            Name = name;
+            Favourites = favourites;
+        }
+
+        public string Name { get; }
+        public IReadOnlyList<IceCream> Favourites { get; }
+
+        public int ScoopsOf(IceCream flavour)
+        {
+            return 2;
+        }
+    }
+}

ideas/split-loop-with-lambda/exercise3/solution/src/Buyers.cs

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+using System;
+using System.Collections.Generic;
+
+namespace IceCreamBuyers
+{
+    /// <summary>
+    /// Reference end state of exercise 3 (extract lambda, slide, extract method
+    /// with a function parameter — the pure rep of Split Loop with Lambda).
+    /// </summary>
+    public class Buyers
+    {
+        private readonly PriceList priceList = new();
+        private readonly List<Buyer> buyers = new()
+        {
+            new("Anke", new List<IceCream> { IceCream.Strawberry, IceCream.Vanilla }),
+            new("Bram", new List<IceCream> { IceCream.Chocolate }),
+            new("Celine", new List<IceCream> { IceCream.Vanilla, IceCream.Chocolate }),
+        };
+
+        /// <summary>
+        /// No-arg overload: callers unchanged, the "real" pricing function baked in.
+        /// </summary>
+        public int ShoppingListTotal(string buyerName)
+        {
+            return ShoppingListTotal(ByName(buyerName), priceList.PriceFor);
+        }
+
+        /// <summary>
+        /// The seam: the pricing source is injected as a function.
+        /// Testable: pass any pure function instead of calling the pricing service.
+        /// </summary>
+        public int ShoppingListTotal(Buyer buyer, Func<IceCream, int> priceFor)
+        {
+            var total = 0;
+            foreach (var flavour in buyer.Favourites)
+            {
+                total += buyer.ScoopsOf(flavour) * priceFor(flavour);
+            }
+            return total;
+        }
+
+        private Buyer ByName(string name)
+        {
+            return buyers.Find(b => b.Name == name)
+                ?? throw new InvalidOperationException($"Unknown buyer {name}.");
+        }
+    }
+}

ideas/split-loop-with-lambda/exercise3/solution/src/Buyers.csproj

@@ -0,0 +1,7 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net7.0</TargetFramework>
+  </PropertyGroup>
+
+</Project>

ideas/split-loop-with-lambda/exercise3/solution/src/IceCream.cs

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+namespace IceCreamBuyers
+{
+    public enum IceCream
+    {
+        Strawberry,
+        Chocolate,
+        Vanilla
+    }
+}

ideas/split-loop-with-lambda/exercise3/solution/src/PriceList.cs

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+using System;
+using System.Collections.Generic;
+
+namespace IceCreamBuyers
+{
+    public class PriceList
+    {
+        // placeholder implementation - real version would call the pricing service
+        private static readonly Dictionary<IceCream, int> BasePrices =
+            new()
+            {
+                { IceCream.Strawberry, 3 },
+                { IceCream.Vanilla, 2 },
+                { IceCream.Chocolate, 4 },
+            };
+
+        public int PriceFor(IceCream flavour)
+        {
+            return BasePrices[flavour] + SurgePricing();
+        }
+
+        private int SurgePricing()
+        {
+            return new Random().Next(3);
+        }
+    }
+}

ideas/split-loop-with-lambda/exercise3/solution/test/BuyersTests.cs

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+using IceCreamBuyers;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using System.Collections.Generic;
+
+namespace IceCreamBuyers.Tests
+{
+    [TestClass]
+    public class BuyersTests
+    {
+        [TestMethod]
+        public void ShoppingListTotalUsesThePricingWePassIn()
+        {
+            // pure pricing function: everything costs 2.
+            // Anke: 2 flavours x 2 scoops x 2 = 8 — deterministic, glanceable.
+            var buyers = new Buyers();
+            var anke = new Buyer("Anke", new List<IceCream> { IceCream.Strawberry, IceCream.Vanilla });
+            Assert.AreEqual(8, buyers.ShoppingListTotal(anke, _ => 2));
+        }
+    }
+}

ideas/split-loop-with-lambda/exercise3/solution/test/BuyersTests.csproj

@@ -0,0 +1,18 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net7.0</TargetFramework>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.7.1" />
+    <PackageReference Include="MSTest.TestAdapter" Version="2.1.1" />
+    <PackageReference Include="MSTest.TestFramework" Version="2.1.1" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\src\Buyers.csproj" />
+  </ItemGroup>
+
+</Project>