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