module Puppet::Functions

Functions in the puppet language can be written in Ruby and distributed in puppet modules. The function is written by creating a file in the module's `lib/puppet/functions/<modulename>` directory, where `<modulename>` is replaced with the module's name. The file should have the name of the function. For example, to create a function named `min` in a module named `math` create a file named `lib/puppet/functions/math/min.rb` in the module.

A function is implemented by calling {Puppet::Functions.create_function}, and passing it a block that defines the implementation of the function.

Functions are namespaced inside the module that contains them. The name of the function is prefixed with the name of the module. For example, `math::min`.

@example A simple function

Puppet::Functions.create_function('math::min') do
  def min(a, b)
    a <= b ? a : b
  end
end

Anatomy of a function


Functions are composed of four parts: the name, the implementation methods, the signatures, and the dispatches.

The name is the string given to the {Puppet::Functions.create_function} method. It specifies the name to use when calling the function in the puppet language, or from other functions.

The implementation methods are ruby methods (there can be one or more) that provide that actual implementation of the function's behavior. In the simplest case the name of the function (excluding any namespace) and the name of the method are the same. When that is done no other parts (signatures and dispatches) need to be used.

Signatures are a way of specifying the types of the function's parameters. The types of any arguments will be checked against the types declared in the signature and an error will be produced if they don't match. The types are defined by using the same syntax for types as in the puppet language.

Dispatches are how signatures and implementation methods are tied together. When the function is called, puppet searches the signatures for one that matches the supplied arguments. Each signature is part of a dispatch, which specifies the method that should be called for that signature. When a matching signature is found, the corresponding method is called.

Special dispatches designed to create error messages for an argument mismatch can be added using the keyword `argument_mismatch` instead of `dispatch`. The method appointed by an `argument_mismatch` will be called with arguments just like a normal `dispatch` would, but the method must produce a string. The string is then used as the message in the `ArgumentError` that is raised when the method returns. A block parameter can be given, but it is not propagated in the method call.

Documentation for the function should be placed as comments to the implementation method(s).

@todo Documentation for individual instances of these new functions is not

yet tied into the puppet doc system.

@example Dispatching to different methods by type

Puppet::Functions.create_function('math::min') do
  dispatch :numeric_min do
    param 'Numeric', :a
    param 'Numeric', :b
  end

  dispatch :string_min do
    param 'String', :a
    param 'String', :b
  end

  def numeric_min(a, b)
    a <= b ? a : b
  end

  def string_min(a, b)
    a.downcase <= b.downcase ? a : b
  end
end

@example Using an argument mismatch handler

Puppet::Functions.create_function('math::min') do
  dispatch :numeric_min do
    param 'Numeric', :a
    param 'Numeric', :b
  end

  argument_mismatch :on_error do
    param 'Any', :a
    param 'Any', :b
  end

  def numeric_min(a, b)
    a <= b ? a : b
  end

  def on_error(a, b)
    'both arguments must be of type Numeric'
  end
end

Specifying Signatures


If nothing is specified, the number of arguments given to the function must be the same as the number of parameters, and all of the parameters are of type 'Any'.

The following methods can be used to define a parameter

- _param_ - the argument must be given in the call.
- _optional_param_ - the argument may be missing in the call. May not be followed by a required parameter
- _repeated_param_ - the type specifies a repeating type that occurs 0 to "infinite" number of times. It may only appear last or just before a block parameter.
- _block_param_ - a block must be given in the call. May only appear last.
- _optional_block_param_ - a block may be given in the call. May only appear last.

The method name required_param is an alias for param and required_block_param is an alias for block_param

A parameter definition takes 2 arguments:

- _type_ A string that must conform to a type in the puppet language
- _name_ A symbol denoting the parameter name

Both arguments are optional when defining a block parameter. The type defaults to “Callable” and the name to :block.

Note that the dispatch definition is used to match arguments given in a call to the function with the defined parameters. It then dispatches the call to the implementation method simply passing the given arguments on to that method without any further processing and it is the responsibility of that method's implementor to ensure that it can handle those arguments.

@example Variable number of arguments

Puppet::Functions.create_function('foo') do
  dispatch :foo do
    param 'Numeric', :first
    repeated_param 'Numeric', :values
  end

  def foo(first, *values)
    # do something
  end
end

There is no requirement for direct mapping between parameter definitions and the parameters in the receiving implementation method so the following example is also legal. Here the dispatch will ensure that `*values` in the receiver will be an array with at least one entry of type String and that any remaining entries are of type Numeric:

@example Inexact mapping or parameters

Puppet::Functions.create_function('foo') do
  dispatch :foo do
    param 'String', :first
    repeated_param 'Numeric', :values
  end

  def foo(*values)
    # do something
  end
end

Access to Scope


In general, functions should not need access to scope; they should be written to act on their given input only. If they absolutely must look up variable values, they should do so via the closure scope (the scope where they are defined) - this is done by calling `closure_scope()`.

Calling other Functions


Calling other functions by name is directly supported via {Puppet::Pops::Functions::Function#call_function}. This allows a function to call other functions visible from its loader.

@api public

Public Class Methods

any_signature(from, to, names) click to toggle source

Construct a signature consisting of Object type, with min, and max, and given names. (there is only one type entry).

@api private

    # File lib/puppet/functions.rb
291 def self.any_signature(from, to, names)
292   # Construct the type for the signature
293   # Tuple[Object, from, to]
294   param_types = Puppet::Pops::Types::PTupleType.new([Puppet::Pops::Types::PAnyType::DEFAULT], Puppet::Pops::Types::PIntegerType.new(from, to))
295   [Puppet::Pops::Types::PCallableType.new(param_types), names]
296 end
create_function(func_name, function_base = Function, &block) click to toggle source

@param func_name [String, Symbol] a simple or qualified function name @param block [Proc] the block that defines the methods and dispatch of the

Function to create

@return [Class<Function>] the newly created Function class

@api public

    # File lib/puppet/functions.rb
184 def self.create_function(func_name, function_base = Function, &block)
185   # Ruby < 2.1.0 does not have method on Binding, can only do eval
186   # and it will fail unless protected with an if defined? if the local
187   # variable does not exist in the block's binder.
188   #
189   begin
190     loader = block.binding.eval('loader_injected_arg if defined?(loader_injected_arg)')
191     create_loaded_function(func_name, loader, function_base, &block)
192   rescue StandardError => e
193     raise ArgumentError, _("Function Load Error for function '%{function_name}': %{message}") % {function_name: func_name, message: e.message}
194   end
195 end
create_loaded_function(func_name, loader, function_base = Function, &block) click to toggle source

Creates a function in, or in a local loader under the given loader. This method should only be used when manually creating functions for the sake of testing. Functions that are autoloaded should always use the `create_function` method and the autoloader will supply the correct loader.

@param func_name [String, Symbol] a simple or qualified function name @param loader [Puppet::Pops::Loaders::Loader] the loader loading the function @param block [Proc] the block that defines the methods and dispatch of the

Function to create

@return [Class<Function>] the newly created Function class

@api public

    # File lib/puppet/functions.rb
210 def self.create_loaded_function(func_name, loader, function_base = Function, &block)
211   if function_base.ancestors.none? { |s| s == Puppet::Pops::Functions::Function }
212     raise ArgumentError, _("Functions must be based on Puppet::Pops::Functions::Function. Got %{function_base}") % { function_base: function_base }
213   end
214 
215   func_name = func_name.to_s
216   # Creates an anonymous class to represent the function
217   # The idea being that it is garbage collected when there are no more
218   # references to it.
219   #
220   # (Do not give the class the block here, as instance variables should be set first)
221   the_class = Class.new(function_base)
222 
223   unless loader.nil?
224     the_class.instance_variable_set(:'@loader', loader.private_loader)
225   end
226 
227   # Make the anonymous class appear to have the class-name <func_name>
228   # Even if this class is not bound to such a symbol in a global ruby scope and
229   # must be resolved via the loader.
230   # This also overrides any attempt to define a name method in the given block
231   # (Since it redefines it)
232   #
233   # TODO, enforce name in lower case (to further make it stand out since Ruby
234   # class names are upper case)
235   #
236   the_class.instance_eval do
237     @func_name = func_name
238 
239     def name
240       @func_name
241     end
242 
243     def loader
244       @loader
245     end
246   end
247 
248   # The given block can now be evaluated and have access to name and loader
249   #
250   the_class.class_eval(&block)
251 
252   # Automatically create an object dispatcher based on introspection if the
253   # loaded user code did not define any dispatchers. Fail if function name
254   # does not match a given method name in user code.
255   #
256   if the_class.dispatcher.empty?
257     simple_name = func_name.split(/::/)[-1]
258     type, names = default_dispatcher(the_class, simple_name)
259     last_captures_rest = (type.size_range[1] == Float::INFINITY)
260     the_class.dispatcher.add(Puppet::Pops::Functions::Dispatch.new(type, simple_name, names, last_captures_rest))
261   end
262 
263   # The function class is returned as the result of the create function method
264   the_class
265 end
default_dispatcher(the_class, func_name) click to toggle source

Creates a default dispatcher configured from a method with the same name as the function

@api private

    # File lib/puppet/functions.rb
270 def self.default_dispatcher(the_class, func_name)
271   unless the_class.method_defined?(func_name)
272     raise ArgumentError, _("Function Creation Error, cannot create a default dispatcher for function '%{func_name}', no method with this name found") % { func_name: func_name }
273   end
274   any_signature(*min_max_param(the_class.instance_method(func_name)))
275 end
min_max_param(method) click to toggle source

@api private

    # File lib/puppet/functions.rb
278 def self.min_max_param(method)
279   result = {:req => 0, :opt => 0, :rest => 0 }
280   # count per parameter kind, and get array of names
281   names = method.parameters.map { |p| result[p[0]] += 1 ; p[1].to_s }
282   from = result[:req]
283   to = result[:rest] > 0 ? :default : from + result[:opt]
284   [from, to, names]
285 end

Public Instance Methods

loader() click to toggle source
    # File lib/puppet/functions.rb
243 def loader
244   @loader
245 end
name() click to toggle source
    # File lib/puppet/functions.rb
239 def name
240   @func_name
241 end