in src/main/scala/lang/Parser.scala [177:191]
private def check(tokenType: TokenType) =
if (isAtEnd) false else peek().tokenType == tokenType
private def isAtEnd = peek().tokenType == EOF
private def peek() = tokens(current)
private def advance() =
if (!isAtEnd) current = current + 1
previous()
private def consume(tokenType: TokenType, message: String = "") = {
if (check(tokenType)) advance()
else throw error(message)
}