Initial commit: LangChain evolution tutorial deck (132 slides)

- Cover, TOC, 5 dividers, 3 recap slides
- 5 sections (chains, langgraph, deepagents, openswe, ecosystem)
- design-system.js with theme tokens + 9 helper functions
- research/: timeline + sources + per-tech notes
- final-compile.js + merge.js for rebuild pipeline
- output/: langchain-evolution.pptx (2.3 MB) + langchain-evolution.pdf (1.1 MB) + 7 sample previews

slides/section5-ecosystem/slide-04.js

@@ -0,0 +1,55 @@
+// Slide 04: @traceable decorator -- the simplest way to instrument any function
+// Code slide: nested decorator pattern.
+
+const ds = require('./design-system');
+
+function createSlide(pres, theme) {
+  const slide = pres.addSlide();
+  ds.helpers.slideBase(slide, pres, theme);
+
+  ds.helpers.addHeader(slide, pres, theme, {
+    eyebrow: 'STAGE 5: ECOSYSTEM',
+    section: 'Tracing',
+    title: '@traceable -- декоратор для любой функции',
+    sectionNumber: 5,
+    titleSize: 24,
+  });
+
+  ds.helpers.addCodeBlock(slide, pres, theme, {
+    x: 0.5, y: 1.45, w: 9.0, h: 3.1,
+    language: 'python',
+    fontSize: 10,
+    code: [
+      'from langsmith import traceable',
+      '',
+      '# Декоратор превращает функцию в traced Run.',
+      '# Все аргументы и return попадают в trace автоматически.',
+      '@traceable(name="retrieve_docs", run_type="retriever")',
+      'def retrieve(query: str, k: int = 4) -> list[str]:',
+      '    return vector_store.similarity_search(query, k=k)',
+      '',
+      '@traceable(name="generate_answer", run_type="chain")',
+      'def generate_answer(query: str) -> str:',
+      '    docs = retrieve(query)                     # nested Run',
+      '    context = "\\n".join(docs)',
+      '    return llm.invoke(f"Q: {query}\\nCtx: {context}")',
+      '',
+      '# Дерево: generate_answer -> retrieve_docs -> ChatModel',
+      'print(generate_answer("What is LCEL?"))',
+    ].join('\n'),
+  });
+
+  // Bottom callout
+  ds.helpers.addCallout(slide, pres, theme, {
+    x: 0.5, y: 4.65, w: 9.0, h: 0.4,
+    kind: 'success',
+    text: 'Вложенные вызовы автоматически становятся дочерними Run-ами в дереве трассировки.',
+  });
+
+  ds.helpers.addSourceLine(slide, pres, theme, {
+    source: 'docs.smith.langchain.com Observability > Log traces > @traceable',
+  });
+  ds.helpers.addPageNumber(slide, pres, theme, 4);
+}
+
+module.exports = { createSlide };