The lookup function syntax has three specific styles of usage, as follows:

• With mandatory <name> and set of three optional arguments: <value type>, <merge behavior> and <default value> in that given order and separated by commas. For example, lookup( <name>, [<value type>], [<merge behavior>], [<default value>] ).
• With optional <name>, and mandatory <options hash> arguments. For example, lookup( [<name>], <options hash> ).
• With mandatory <name> and <lambda expression> arguments. For example, lookup( <name>, <lambda expression> ).