Merge branch 'main' of ssh://gitea.apps.sustainabledelivery.com:3022/learning-hours/learning-hour-maker

Verified with SDK 10.0.4xx (mise): builds clean, solution test green
deterministically, pristine test flaky as designed (failed 2/3 runs).
net7.0 runtime is out of support and unavailable, so the exercise now
targets net10.0; MSTest bumped 2.1.1 -> 3.6.4 to silence the NU1903
transitive-dependency warning. .gitignore for bin/obj.

ideas/split-loop-with-lambda/exercise3/.gitignore

@@ -0,0 +1,3 @@
+bin/
+obj/
+*.user

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

@@ -0,0 +1,42 @@
+# 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 10; verified with `dotnet test` on SDK
+10.0.4xx — pristine test is flaky by design, solution is deterministic).
+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.
+
+## Verified
+
+Built and run with `dotnet test` (SDK 10.0.4xx): solution green deterministically,
+pristine test flaky (failed 2 of 3 runs, passed 1 — both surges rolled 0).
+MSTest bumped to 3.6.4 (kata ships 2.1.1, whose transitive Newtonsoft.Json trips
+a NU1903 vulnerability warning).

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

@@ -0,0 +1,7 @@
+{
+  "sdk": {
+    "version": "10.0.0",
+    "rollForward": "latestFeature",
+    "allowPrerelease": true
+  }
+}

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

@@ -0,0 +1,33 @@
+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

@@ -0,0 +1,7 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+  </PropertyGroup>
+
+</Project>

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

@@ -0,0 +1,9 @@
+namespace IceCreamBuyers
+{
+    public enum IceCream
+    {
+        Strawberry,
+        Chocolate,
+        Vanilla
+    }
+}

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

@@ -0,0 +1,27 @@
+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

@@ -0,0 +1,19 @@
+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

@@ -0,0 +1,18 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.11.1" />
+    <PackageReference Include="MSTest.TestAdapter" Version="3.6.4" />
+    <PackageReference Include="MSTest.TestFramework" Version="3.6.4" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\src\Buyers.csproj" />
+  </ItemGroup>
+
+</Project>

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

@@ -0,0 +1,50 @@
+# 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": "10.0.0",
+    "rollForward": "latestFeature",
+    "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

@@ -0,0 +1,48 @@
+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>net10.0</TargetFramework>
+  </PropertyGroup>
+
+</Project>

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

@@ -0,0 +1,9 @@
+namespace IceCreamBuyers
+{
+    public enum IceCream
+    {
+        Strawberry,
+        Chocolate,
+        Vanilla
+    }
+}

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

@@ -0,0 +1,27 @@
+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

@@ -0,0 +1,20 @@
+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>net10.0</TargetFramework>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.11.1" />
+    <PackageReference Include="MSTest.TestAdapter" Version="3.6.4" />
+    <PackageReference Include="MSTest.TestFramework" Version="3.6.4" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\src\Buyers.csproj" />
+  </ItemGroup>
+
+</Project>

ideas/split-loop-with-lambda/storyboard/buyers-example.md

@@ -0,0 +1,126 @@
+# Example 3 — Buyers: extract lambda in its purest form
+
+Third rep of the Split Loop with Lambda mechanics, in C#. Example 1
+(`longRangeForecast`) had a drift trap and a dead call; the demo
+(`printSalesForecasts`) had three dependencies and grew into a role interface.
+This one has **one loop and one lambda-able expression, and nothing else** —
+the move in its purest form. Diagrams use the same
+[vocabulary as the main storyboard](storyboard.md).
+
+Kata theme: the sellers exist (`DailyForecasts` — Steve, Julie, Francis), the
+buyers don't. `Buyers` computes a buyer's shopping-list total; `PriceList`
+stands for the external pricing service. Exercise repo: `../exercise3/csharp`,
+end state: `../exercise3/solution`.
+
+---
+
+## Step 0 — one loop, one unwanted dependency
+
+**Goal:** the test is red or flaky — why? (`PriceList` adds random surge
+pricing; it stands for the external pricing service.)
+
+![](svg/b0-start.svg)
+
+**Before:**
+
+```csharp
+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);  // ← unwanted dependency
+    }
+    return total;
+}
+```
+
+**Checkpoint:** run the test. Flaky. What does the loop need from outside?
+*Given a flavour, what does it cost?* That question is the lambda.
+
+---
+
+## Step 1 — extract the pricing call into a lambda, and slide it
+
+**Goal:** the move, with nothing else in the way. Extract, then slide — the
+invocation stays in the loop.
+
+![](svg/b1-lambda-slide.svg)
+
+**After:**
+
+```csharp
+public int ShoppingListTotal(string buyerName)
+{
+    Func<IceCream, int> priceFor = flavour => priceList.PriceFor(flavour);  // ← SLID to the top...
+    var buyer = ByName(buyerName);
+    var total = 0;
+    foreach (var flavour in buyer.Favourites)
+    {
+        total += buyer.ScoopsOf(flavour) * priceFor(flavour);               // ← ...invocation stays here
+    }
+    return total;
+}
+```
+
+**Checkpoint:** compiles; test still flaky (the lambda still calls
+`PriceList` — that is fine). Unlike the scoring-loop example, `priceList` is a
+*field*, so the lambda slides all the way to the top. Slide, don't move.
+
+---
+
+## Step 2 — extract the loop; the lambda becomes a parameter
+
+**Goal:** select the loop, Extract Method. The loop touches no fields — the
+lambda is the only thing crossing the boundary.
+
+![](svg/b2-extract-test.svg)
+
+**After:**
+
+```csharp
+public int ShoppingListTotal(string buyerName)                           // ← no-arg overload: callers unchanged
+{
+    return ShoppingListTotal(ByName(buyerName), priceList.PriceFor);     // ← the "real" pricing, as a method group
+}
+
+public int ShoppingListTotal(Buyer buyer, Func<IceCream, int> priceFor)  // ← the seam: pricing is a parameter
+{
+    var total = 0;
+    foreach (var flavour in buyer.Favourites)
+    {
+        total += buyer.ScoopsOf(flavour) * priceFor(flavour);
+    }
+    return total;
+}
+```
+
+(The extracted loop method touches no fields; it sits in the same class as
+`ByName` for convenience. Move it to a class without `PriceList` any time —
+that is step 4 of the full pattern, unblocked.)
+
+**The test — the whole point:**
+
+```csharp
+[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));
+}
+```
+
+**Checkpoint:** green — deterministically. `PriceList` is never touched.
+
+- `grep PriceFor src/` — the extracted method has no hits.
+- C# note: the extracted method stays an instance method here because the
+  class also holds `ByName`; if you want a `static`, extract the loop only and
+  keep the lookup in the composed method.
+
+**Debrief in one line:** the loop now reads "for each favourite flavour, add
+scoops times price" — and the pricing is a question anyone can answer with one
+line of test code.

ideas/split-loop-with-lambda/storyboard/svg/b0-start.svg

@@ -0,0 +1,38 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 760 460" font-family="Helvetica, Arial, sans-serif">
+  <defs>
+    <marker id="arrR" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse">
+      <path d="M0,0 L10,5 L0,10 z" fill="#dc2626"/>
+    </marker>
+  </defs>
+  <rect width="760" height="460" fill="#ffffff"/>
+  <text x="24" y="34" font-size="20" font-weight="bold" fill="#0f172a">Step 0 — one loop, one unwanted dependency</text>
+  <text x="24" y="56" font-size="13" fill="#475569">PriceList stands for the external pricing service (random surge pricing) — that is why the test is red &amp; flaky.</text>
+
+  <!-- method -->
+  <rect x="60" y="74" width="400" height="260" rx="10" fill="#f8fafc" stroke="#64748b" stroke-width="1.5"/>
+  <text x="76" y="98" font-size="14" font-weight="bold" fill="#0f172a">ShoppingListTotal(buyerName)</text>
+
+  <text x="80" y="126" font-size="11" font-family="Menlo, Consolas, monospace" fill="#1e1b4b">buyer = ByName(buyerName)</text>
+
+  <!-- loop -->
+  <rect x="80" y="140" width="360" height="120" rx="8" fill="#eef2ff" stroke="#818cf8"/>
+  <text x="94" y="166" font-size="11" font-family="Menlo, Consolas, monospace" fill="#1e1b4b">foreach (var flavour : buyer.Favourites)</text>
+  <text x="94" y="196" font-size="11" font-family="Menlo, Consolas, monospace" fill="#1e1b4b">total += buyer.ScoopsOf(flavour) *</text>
+  <text x="94" y="216" font-size="11" font-family="Menlo, Consolas, monospace" font-weight="bold" fill="#dc2626">  priceList.PriceFor(flavour);</text>
+  <text x="94" y="244" font-size="10" fill="#64748b">the pricing loop — one expression, nothing else</text>
+
+  <text x="80" y="300" font-size="11" font-family="Menlo, Consolas, monospace" fill="#1e1b4b">return total;</text>
+
+  <!-- PriceList -->
+  <rect x="580" y="150" width="150" height="90" rx="8" fill="#fef2f2" stroke="#dc2626" stroke-width="1.5"/>
+  <text x="655" y="180" font-size="14" font-weight="bold" fill="#dc2626" text-anchor="middle">PriceList</text>
+  <text x="655" y="200" font-size="11" fill="#7f1d1d" text-anchor="middle">stands for the pricing</text>
+  <text x="655" y="216" font-size="11" fill="#7f1d1d" text-anchor="middle">service — Random surge</text>
+
+  <line x1="444" y1="206" x2="572" y2="192" stroke="#dc2626" stroke-width="1.5" stroke-dasharray="5 4" marker-end="url(#arrR)"/>
+
+  <!-- test -->
+  <rect x="580" y="290" width="150" height="60" rx="8" fill="#ffffff" stroke="#94a3b8" stroke-dasharray="4 3"/>
+  <text x="655" y="314" font-size="12" fill="#334155" text-anchor="middle">test: Anke = 10?</text>
+  <text x="655" y="332" font-size="12" font-weight="bold" fill="#dc2626" text-anchor="middle">red &amp; flaky ✗</text>
+</svg>

ideas/split-loop-with-lambda/storyboard/svg/b1-lambda-slide.svg

@@ -0,0 +1,50 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 760 460" font-family="Helvetica, Arial, sans-serif">
+  <defs>
+    <marker id="arrR" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse">
+      <path d="M0,0 L10,5 L0,10 z" fill="#dc2626"/>
+    </marker>
+    <marker id="arrP" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse">
+      <path d="M0,0 L10,5 L0,10 z" fill="#9333ea"/>
+    </marker>
+  </defs>
+  <rect width="760" height="460" fill="#ffffff"/>
+  <text x="24" y="34" font-size="20" font-weight="bold" fill="#0f172a">Step 1 — extract the pricing call into a lambda, and slide it</text>
+  <text x="24" y="56" font-size="13" fill="#9333ea" font-weight="bold">One expression, nothing else in the way. priceList is a field, so the lambda slides all the way to the top.</text>
+
+  <!-- method -->
+  <rect x="60" y="74" width="400" height="330" rx="10" fill="#f8fafc" stroke="#64748b" stroke-width="1.5"/>
+  <text x="76" y="98" font-size="14" font-weight="bold" fill="#0f172a">ShoppingListTotal(buyerName)</text>
+
+  <!-- lambda at top -->
+  <rect x="80" y="110" width="360" height="82" rx="8" fill="#faf5ff" stroke="#9333ea" stroke-width="1.5"/>
+  <text x="94" y="136" font-size="12" font-weight="bold" fill="#9333ea">λ priceFor</text>
+  <text x="94" y="162" font-size="11" font-family="Menlo, Consolas, monospace" fill="#dc2626">Func&lt;IceCream,int&gt; =</text>
+  <text x="94" y="180" font-size="11" font-family="Menlo, Consolas, monospace" fill="#dc2626">  flavour =&gt; priceList.PriceFor(flavour);</text>
+
+  <text x="80" y="200" font-size="11" font-family="Menlo, Consolas, monospace" fill="#1e1b4b">buyer = ByName(buyerName)</text>
+
+  <!-- loop -->
+  <rect x="80" y="214" width="360" height="120" rx="8" fill="#eef2ff" stroke="#818cf8"/>
+  <text x="94" y="240" font-size="11" font-family="Menlo, Consolas, monospace" fill="#1e1b4b">foreach (var flavour : buyer.Favourites)</text>
+  <text x="94" y="270" font-size="11" font-family="Menlo, Consolas, monospace" fill="#1e1b4b">total += buyer.ScoopsOf(flavour) *</text>
+  <text x="94" y="290" font-size="11" font-family="Menlo, Consolas, monospace" font-weight="bold" fill="#6b21a8">  priceFor(flavour);</text>
+  <text x="94" y="318" font-size="10" fill="#64748b">invocation stays exactly where the code was</text>
+
+  <!-- ghost -->
+  <rect x="94" y="352" width="330" height="26" rx="6" fill="none" stroke="#9333ea" stroke-width="1" stroke-dasharray="4 3"/>
+  <text x="106" y="370" font-size="10" fill="#9333ea" font-style="italic">λ used to be defined right here</text>
+
+  <text x="80" y="398" font-size="11" font-family="Menlo, Consolas, monospace" fill="#1e1b4b">return total;</text>
+
+  <!-- slide arrow -->
+  <line x1="260" y1="350" x2="260" y2="196" stroke="#9333ea" stroke-width="1.5" stroke-dasharray="5 4" marker-end="url(#arrP)"/>
+  <text x="270" y="205" font-size="12" font-weight="bold" fill="#9333ea">slide</text>
+
+  <!-- PriceList -->
+  <rect x="580" y="130" width="150" height="90" rx="8" fill="#fef2f2" stroke="#dc2626" stroke-width="1.5"/>
+  <text x="655" y="160" font-size="14" font-weight="bold" fill="#dc2626" text-anchor="middle">PriceList</text>
+  <text x="655" y="180" font-size="11" fill="#7f1d1d" text-anchor="middle">still called —</text>
+  <text x="655" y="196" font-size="11" fill="#7f1d1d" text-anchor="middle">but only via the λ now</text>
+
+  <line x1="444" y1="142" x2="572" y2="162" stroke="#dc2626" stroke-width="1.5" stroke-dasharray="5 4" marker-end="url(#arrR)"/>
+</svg>

ideas/split-loop-with-lambda/storyboard/svg/b2-extract-test.svg

@@ -0,0 +1,54 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 760 460" font-family="Helvetica, Arial, sans-serif">
+  <defs>
+    <marker id="arrR" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse">
+      <path d="M0,0 L10,5 L0,10 z" fill="#dc2626"/>
+    </marker>
+    <marker id="arrGr" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse">
+      <path d="M0,0 L10,5 L0,10 z" fill="#16a34a"/>
+    </marker>
+  </defs>
+  <rect width="760" height="460" fill="#ffffff"/>
+  <text x="24" y="34" font-size="20" font-weight="bold" fill="#0f172a">Step 2 — extract the loop; the lambda becomes a parameter</text>
+  <text x="24" y="56" font-size="13" fill="#16a34a" font-weight="bold">The test passes its own pricing (_ =&gt; 2). PriceList is never touched: deterministic, glanceable.</text>
+
+  <!-- test -->
+  <rect x="50" y="150" width="230" height="140" rx="10" fill="#f0fdf4" stroke="#16a34a" stroke-width="1.5"/>
+  <text x="66" y="176" font-size="13" font-weight="bold" fill="#14532d">BuyersTests</text>
+  <text x="66" y="204" font-size="11" font-family="Menlo, Consolas, monospace" fill="#334155">var anke = new Buyer(...)</text>
+  <text x="66" y="228" font-size="12" font-family="Menlo, Consolas, monospace" font-weight="bold" fill="#16a34a">priceFor: _ =&gt; 2</text>
+  <text x="66" y="252" font-size="11" font-family="Menlo, Consolas, monospace" fill="#14532d">ShoppingListTotal(anke, λ)</text>
+  <text x="66" y="276" font-size="11" font-family="Menlo, Consolas, monospace" fill="#14532d">Assert.AreEqual(8, ...)</text>
+
+  <!-- middle: overload -->
+  <rect x="330" y="150" width="200" height="180" rx="10" fill="#f8fafc" stroke="#64748b" stroke-width="1.5"/>
+  <text x="344" y="176" font-size="11" font-weight="bold" fill="#0f172a">ShoppingListTotal(buyer, <tspan fill="#9333ea">λ</tspan>)</text>
+  <text x="344" y="196" font-size="10" fill="#475569">no-arg overload passes</text>
+  <text x="344" y="210" font-size="10" fill="#475569">priceList.PriceFor as method group</text>
+  <text x="344" y="238" font-size="11" font-family="Menlo, Consolas, monospace" fill="#1e1b4b">total = 0</text>
+  <text x="344" y="262" font-size="11" font-family="Menlo, Consolas, monospace" fill="#1e1b4b">foreach flavour:</text>
+  <text x="344" y="286" font-size="11" font-family="Menlo, Consolas, monospace" fill="#1e1b4b">total += scoops *</text>
+  <text x="344" y="306" font-size="11" font-family="Menlo, Consolas, monospace" fill="#1e1b4b"><tspan fill="#9333ea" font-weight="bold">λ</tspan>(flavour)</text>
+
+  <!-- right: the extracted loop, pure -->
+  <rect x="570" y="150" width="170" height="180" rx="10" fill="#f8fafc" stroke="#64748b" stroke-width="1.5"/>
+  <text x="584" y="176" font-size="10" font-weight="bold" fill="#0f172a">the pricing loop</text>
+  <text x="584" y="196" font-size="10" font-family="Menlo, Consolas, monospace" fill="#1e1b4b">inputs: buyer, λ</text>
+  <text x="584" y="220" font-size="10" font-family="Menlo, Consolas, monospace" fill="#1e1b4b">sum of scoops *</text>
+  <text x="584" y="238" font-size="10" font-family="Menlo, Consolas, monospace" fill="#1e1b4b"><tspan fill="#9333ea" font-weight="bold">λ</tspan>(flavour)</text>
+  <text x="584" y="270" font-size="10" font-family="Menlo, Consolas, monospace" fill="#1e1b4b">return total</text>
+  <text x="584" y="304" font-size="10" fill="#16a34a" font-weight="bold">pure ✓</text>
+
+  <!-- green arrows -->
+  <line x1="284" y1="225" x2="324" y2="225" stroke="#16a34a" stroke-width="1.5" marker-end="url(#arrGr)"/>
+  <text x="286" y="214" font-size="10" fill="#16a34a">λ</text>
+  <line x1="534" y1="225" x2="564" y2="225" stroke="#16a34a" stroke-width="1.5" marker-end="url(#arrGr)"/>
+
+  <!-- PriceList, never called -->
+  <rect x="570" y="370" width="170" height="56" rx="8" fill="#f8fafc" stroke="#94a3b8" stroke-width="1.5" stroke-dasharray="4 3" opacity="0.8"/>
+  <text x="655" y="392" font-size="12" font-weight="bold" fill="#94a3b8" text-anchor="middle">PriceList</text>
+  <text x="655" y="412" font-size="10" fill="#64748b" text-anchor="middle">never called by the test ✗</text>
+  <line x1="655" y1="368" x2="655" y2="336" stroke="#dc2626" stroke-width="1.5" stroke-dasharray="5 4" marker-end="url(#arrR)"/>
+  <text x="665" y="356" font-size="11" font-weight="bold" fill="#dc2626">✗</text>
+
+  <text x="50" y="430" font-size="12" fill="#475569">2 flavours x 2 scoops x 2 = 8 — every run, not just a lucky roll.</text>
+</svg>