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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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commit 1189041d8e
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ideas/split-loop-with-lambda/storyboard/svg/b0-start.svg
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<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 -->
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<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>
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<!-- 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>
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