Symbol Literals

Remarks#

Scala comes with a concept of symbols - strings that are interned, that is: two symbols with the same name (the same character sequence), in contrary to strings, will refer to the same object during execution.

Symbols are a feature of many languages: Lisp, Ruby and Erlang and more, however in Scala they are of relatively small use. Good feature to have nevertheless.

Use:

Any literal beginning with a single quote ', followed by one or more digits, letters, or under‐scores _ is a symbol literal. The first character is an exception as it can’t be a digit.

Good definitions:

'ATM
'IPv4
'IPv6
'map_to_operations
'data_format_2006

// Using the `Symbol.apply` method

Symbol("hakuna matata")
Symbol("To be or not to be that is a question")

Bad definitions:

'8'th_division
'94_pattern
'bad-format

Replacing strings in case clauses

Let’s say we have multiple data sources which include database, file, prompt and argumentList. Depending on chosen source we change our approach:

def loadData(dataSource: Symbol): Try[String] = dataSource match {
  case 'database => loadDatabase() // Loading data from database
  case 'file =>  loadFile() // Loading data from file
  case 'prompt => askUser() // Asking user for data
  case 'argumentList => argumentListExtract() // Accessing argument list for data
  case _ => Failure(new Exception("Unsupported data source"))
}

We could have very well used String in place of Symbol. We didn’t, because none of strings’s features are useful in this context.

This makes the code simpler and less error prone.