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node_modules/graphql/validation/rules/KnownFragmentNames.js.flow

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+// @flow strict
+
+import { GraphQLError } from '../../error/GraphQLError';
+import { type ASTVisitor } from '../../language/visitor';
+
+import { type ValidationContext } from '../ValidationContext';
+
+export function unknownFragmentMessage(fragName: string): string {
+  return `Unknown fragment "${fragName}".`;
+}
+
+/**
+ * Known fragment names
+ *
+ * A GraphQL document is only valid if all `...Fragment` fragment spreads refer
+ * to fragments defined in the same document.
+ */
+export function KnownFragmentNames(context: ValidationContext): ASTVisitor {
+  return {
+    FragmentSpread(node) {
+      const fragmentName = node.name.value;
+      const fragment = context.getFragment(fragmentName);
+      if (!fragment) {
+        context.reportError(
+          new GraphQLError(unknownFragmentMessage(fragmentName), node.name),
+        );
+      }
+    },
+  };
+}