Groovy for Domain-specific Languages - Second Edition

Chapter 5

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Closure parameters

In our previous examples, we have made use of the it keyword. When a closure
accepts only a single parameter, we are able to refer to this parameter as it and are
free from having to explicitly define the parameter. The possible syntax definitions
for a closure are:

• The default case allows any parameters to be passed to the closure:
  {
  // statements
  }


• The closure does not accept any parameters:
  { ->
  // statements
  }


• The closure can accept one to many parameters with optional type annotations:

{ [type] param (,[type] param)* ->

// statements

}

The parameter list is a comma-separated list of parameter names with optional type
definitions. Closures behave slightly different depending on whether we supply the
optional type:

given: "Closures with various parameter definition"

def defaultParams = { println it; }

def dynamicParams = { something -> println something; }

def intParams = { int something -> println something; }

def stringParams = { String something -> println something; }

def noParams = { -> }

when: "Invoking these with valid parameters"

defaultParams 1

defaultParams "String"

dynamicParams 1

dynamicParams "String"

intParams 1

stringParams "String"

noParams ()

then: "we expect them to work"

notThrown MissingMethodException

when: "we pass an incorrect type for a typed parameter"

stringParams 1