flow like the river

BACK_BACK/node_modules/graphql/validation/rules/UniqueFragmentNamesRule.js.flow

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+// @flow strict
+import { GraphQLError } from '../../error/GraphQLError';
+
+import type { ASTVisitor } from '../../language/visitor';
+
+import type { ASTValidationContext } from '../ValidationContext';
+
+/**
+ * Unique fragment names
+ *
+ * A GraphQL document is only valid if all defined fragments have unique names.
+ */
+export function UniqueFragmentNamesRule(
+  context: ASTValidationContext,
+): ASTVisitor {
+  const knownFragmentNames = Object.create(null);
+  return {
+    OperationDefinition: () => false,
+    FragmentDefinition(node) {
+      const fragmentName = node.name.value;
+      if (knownFragmentNames[fragmentName]) {
+        context.reportError(
+          new GraphQLError(
+            `There can be only one fragment named "${fragmentName}".`,
+            [knownFragmentNames[fragmentName], node.name],
+          ),
+        );
+      } else {
+        knownFragmentNames[fragmentName] = node.name;
+      }
+      return false;
+    },
+  };
+}