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

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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.
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2026-09-28 21:48:30 +00:00
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ideas/split-loop-with-lambda/storyboard/svg/b2-extract-test.svg
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<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>
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<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>
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<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>
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<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>
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<text x="655" y="412" font-size="10" fill="#64748b" text-anchor="middle">never called by the test ✗</text>
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<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>
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