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

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+// @flow strict
+
+import { GraphQLError } from '../../error/GraphQLError';
+import { type ASTVisitor } from '../../language/visitor';
+
+import { type SDLValidationContext } from '../ValidationContext';
+
+export function schemaDefinitionNotAloneMessage(): string {
+  return 'Must provide only one schema definition.';
+}
+
+export function canNotDefineSchemaWithinExtensionMessage(): string {
+  return 'Cannot define a new schema within a schema extension.';
+}
+
+/**
+ * Lone Schema definition
+ *
+ * A GraphQL document is only valid if it contains only one schema definition.
+ */
+export function LoneSchemaDefinition(
+  context: SDLValidationContext,
+): ASTVisitor {
+  const oldSchema = context.getSchema();
+  const alreadyDefined =
+    oldSchema &&
+    (oldSchema.astNode ||
+      oldSchema.getQueryType() ||
+      oldSchema.getMutationType() ||
+      oldSchema.getSubscriptionType());
+
+  let schemaDefinitionsCount = 0;
+  return {
+    SchemaDefinition(node) {
+      if (alreadyDefined) {
+        context.reportError(
+          new GraphQLError(canNotDefineSchemaWithinExtensionMessage(), node),
+        );
+        return;
+      }
+
+      if (schemaDefinitionsCount > 0) {
+        context.reportError(
+          new GraphQLError(schemaDefinitionNotAloneMessage(), node),
+        );
+      }
+      ++schemaDefinitionsCount;
+    },
+  };
+}