add expectOptionalKeyword and optionalMany

Author: samisuteria

Sources/GraphQL/Language/Parser.swift

@@ -1342,6 +1342,20 @@ func expectKeyword(lexer: Lexer, value: String) throws -> Token {
     return token
 }
 
+/**
+* If the next token is a given keyword, return "true" after advancing the lexer.
+* Otherwise, do not change the parser state and return "false".
+*/
+@discardableResult
+func expectOptionalKeyword(lexer: Lexer, value: String) throws -> Bool {
+    let token = lexer.token
+    guard token.kind == .name && token.value == value else {
+        return false
+    }
+    try lexer.advance()
+    return true
+}
+
 /**
  * Helper func for creating an error when an unexpected lexed token
  * is encountered.
@@ -1377,6 +1391,23 @@ func any<T>(
     return nodes
 }
 
+/**
+* Returns a list of parse nodes, determined by the parseFn.
+* It can be empty only if open token is missing otherwise it will always return non-empty list
+* that begins with a lex token of openKind and ends with a lex token of closeKind.
+* Advances the parser to the next lex token after the closing token.
+*/
+func optionalMany<T>(lexer: Lexer, openKind: Token.Kind, closeKind: Token.Kind, parse: (Lexer) throws -> T) throws -> [T] {
+    guard try expectOptional(lexer: lexer, kind: openKind) != nil else {
+        return []
+    }
+    var nodes: [T] = []
+    while try !skip(lexer: lexer, kind: closeKind) {
+        nodes.append(try parse(lexer))
+    }
+    return nodes
+}
+
 /**
  * Returns a non-empty list of parse nodes, determined by
  * the parseFn. This list begins with a lex token of openKind