feat: Adds concatAST

Author: NeedleInAJayStack

Sources/GraphQL/Utilities/ConcatAST.swift

@@ -0,0 +1,14 @@
+/**
+ * Provided a collection of ASTs, presumably each from different files,
+ * concatenate the ASTs together into batched AST, useful for validating many
+ * GraphQL source files which together represent one conceptual application.
+ */
+func concatAST(
+    documents: [Document]
+) -> Document {
+    var definitions: [Definition] = []
+    for doc in documents {
+        definitions.append(contentsOf: doc.definitions)
+    }
+    return Document(definitions: definitions)
+}

Tests/GraphQLTests/UtilitiesTests/ConcatASTTests.swift

@@ -0,0 +1,35 @@
+@testable import GraphQL
+import XCTest
+
+class ConcatASTTests: XCTestCase {
+    func testConcatenatesTwoASTsTogether() throws {
+        let sourceA = Source(body: """
+        { a, b, ...Frag }
+        """)
+
+        let sourceB = Source(body: """
+        fragment Frag on T {
+          c
+        }
+        """)
+
+        let astA = try parse(source: sourceA)
+        let astB = try parse(source: sourceB)
+        let astC = concatAST(documents: [astA, astB])
+
+        XCTAssertEqual(
+            print(ast: astC),
+            """
+            {
+              a
+              b
+              ...Frag
+            }
+
+            fragment Frag on T {
+              c
+            }
+            """
+        )
+    }
+}