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