learning-hour-maker/exercises/slice-with-lambda/README.md

Slice with Lambda — C# exercises

Exercises for the Slice with Lambda learning hour (C# variant).

• Condensed mechanics: https://www.sammancoaching.org/refactorings/slice.html (where that page says mock, we use a lambda instead).
• Full Java walkthrough: ../../ideas/slice-with-lambda/storyboard/storyboard.md.

Open SliceWithLambda.sln in Rider (or run dotnet test — see below). Requires the .NET 10 SDK (pinned by global.json).

What's in the box

folder	what	start state
src/SliceWithLambda/	three demo examples (pristine)	the refactoring not done yet
src/Buyers/	the pair exercise (pristine)	one loop, one lambda-able expression
solution/	reference end states (facilitator + fast pairs)	mechanics done, tests stub through the seam
tests/	test projects	see the red/green table

The three demo examples (src/SliceWithLambda/)

file	guard (client state)	unwanted dependency	lambda	tail
MetaphorFree.cs	if (a < 0) return 0;	_someDependency.mightThrowWhenPreconditionNotSatisfied(a)	Func<int, int> readB = a => ...	int c = a + b; return c * 2;
WeatherStation.cs	if (_wateredToday)	_lightSensor.Read() (byte)	Func<byte> readLight = () => ...	sunny -> WateringDuration.Medium, shady -> Short (reading > 128 => SunExposure.Sunny)
IceCreamShop.cs	if (_thawedToday)	_thermometer.Read() (byte)	Func<byte> readTemperature = () => ...	warm -> ThawAmount.Medium, cold -> Low (temperature > 25 => Warmth.Warm)

In all three, the guard reads client state (a parameter or a flag the client owns), so it stays in the extracted method. Only the dependency call moves into the lambda.

MetaphorFree.cs is the smallest possible shape: no loop, one dependency call, a tail of one arithmetic step. It carries the capture-to-parameter lesson: the lambda takes a as its parameter because a lambda moved up into the function head cannot capture method parameters. WeatherStation.cs and IceCreamShop.cs are same-shape domain repetitions, so participants can practice extracting the lambda twice — the C# delegate syntax (Func<byte>) is worth the repetition.

The pair exercise (src/Buyers/)

Buyers.ShoppingListTotal(buyerName) computes a buyer's shopping-list total: one loop, and exactly one lambda-able expression — priceList.PriceFor(flavour). PriceList adds random surge pricing, so the test in tests/BuyersTests/ is red or flaky by design: it stands for the external pricing service.

Do the mechanics on the whole method:

1. Extract the dependency-using code into a lambda and slide it to the top (Func<IceCream, int> priceFor = flavour => priceList.PriceFor(flavour); — the invocation in the loop stays put).
2. Extract the loop into a method taking the lambda as a parameter.
3. Keep a no-arg overload for callers (pass the method group priceList.PriceFor as the "real" pricing function) and make the test pass with a pure lambda (_ => 2): 2 flavours x 2 scoops x 2 = 8.

Reference end state: solution/Buyers/ (green test in tests/BuyersSolutionTests/).

Tests: red/green map

test project	against	expected at start	after the refactoring
tests/BuyersTests/	src/Buyers/	red or flaky (surge pricing) — that is the point	green once you stub with a pure lambda
tests/BuyersSolutionTests/	solution/Buyers/	green	—
tests/SliceWithLambdaTests/	solution/SliceWithLambda/	green	— (the demo end states, with pure-lambda stubs, incl. a throwing stub)

Run

dotnet test                                    # everything
dotnet test tests/BuyersTests                  # the pair exercise only

Next move (debrief)

Add DiscountFor(IceCream) to PriceList and feel why one lambda is no longer enough — extract a role interface (IPricing) instead of two lambdas.