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packages/api-stack/api/api-graphql/src/federation/parse-sdl.md

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+# `parse-sdl`
+
+## Purpose
+
+Minimal SDL parser used by the federation merger to locate `type Name { ... }`
+and `extend type Name { ... }` blocks plus the field names inside each.
+This is **not** a full GraphQL AST parser — it is the contract-layer
+dependency-free helper that recognises the constructs the federation rules
+care about (design §10.2).
+
+Engine adapters re-parse the merged SDL with their own AST machinery. The
+purpose of this parser is the cross-plugin invariant check before the
+engine ever sees the schema.
+
+## Features
+
+- Recognises `type Name { ... }` and `extend type Name { ... }` blocks.
+- Extracts field names (handles inline argument lists `field(a: Int): T`).
+- Strips line comments (`# ...`) and block string descriptions
+  (`"""..."""`) before parsing.
+- Ignores union / interface / input / enum / scalar declarations — these
+  pass through to the merged SDL but don't contribute to the field-level
+  collision check.
+
+## Usage: Basic
+
+```ts
+import { parseSdl } from "@suaveplan/api-graphql/federation";
+
+const blocks = parseSdl(
+    "type Invoice { id: ID! total: Int! } extend type Query { invoices: [Invoice!]! }"
+);
+console.log(blocks[0].typeName); // "Invoice"
+console.log(blocks[1].kind);     // "extend"
+console.log(blocks[1].fields);   // ["invoices"]
+```
+
+## Usage: Advanced
+
+Strip descriptions before parsing for downstream tooling that wants only
+the structural SDL:
+
+```ts
+import { stripCommentsAndDescriptions } from "@suaveplan/api-graphql/federation";
+
+const cleaned = stripCommentsAndDescriptions(`
+    """Invoice domain root"""
+    type Invoice {
+        # the canonical id
+        id: ID!
+    }
+`);
+```
+
+## API Reference
+
+- `parseSdl(sdl) → TypeBlock[]` — extract structured blocks.
+- `stripCommentsAndDescriptions(sdl) → string` — comment + block-string
+  removal.
+- `TypeBlock` — `{ kind: "type" | "extend"; typeName; fields; rawBlock }`.
+
+## Implementation Notes
+
+The parser uses three regexes: one for block-string descriptions, one for
+line comments, and one for the type/extend block envelope. Field-name
+extraction is line-oriented; multi-line argument lists fall back to the
+inline-argument regex, which is `([a-z_][a-z0-9_]*)\s*(?:\([^)]*\))?\s*:`.
+
+Pathological inputs (nested braces inside field type expressions, e.g.
+`field: { ... }`) are not supported — GraphQL SDL doesn't use them. If a
+future GraphQL spec extension introduces nested braces, the regex would
+need replacing with a proper tokeniser; that's a known limit and acceptable
+for the contract layer.