Parsing inner functions

[yulric]

An inner function is a function declared inside of another one, for example,

function_one <- function() {
    function_two <- function() {

    }
}

In the above R code, function_two is an inner function.

Things to consider when parsing an inner function:

• The name of the inner function: The name should make it clear that this is a function inside of another function and should have the name of the parent function in it. Proposing that we use the dot notation to represent that the inner function is a property of the parent function. For example, in the above example the name of the inner function would be function_one.function_two.
• Handling closure variables: The inner function is able to use variables that are in the parent function's environment. For example,

  function_one <- function(a) {
      function_two <- function(b) {
           return(a + b)
      }
  }
  

  In the above example, function_two is able to use the variable a even though its not declared anywhere in its scope. In PMML however each function has only one scope, its own. To accommodate this R feature when the inner function is parsed into PMML:
  • Any accessed variables that are not in its scope will need to be added as extra parameters to the inner function's signature
  • The names of these new parameters should use the same dot notation as in the inner function's name.
  • Keeping the above points into account, the PMML for function_two would look like,

  <Apply function="function_one.function_two">
      <ParameterField name="b" dataType="double" />
      <ParameterField name="function_one.a" dataType="double" />
      <Apply function="+">
          <FieldRef field="function_one.a"/>>
          <FieldRef field="b"/>
      </Apply>
  </Apply>
  

• Closure variables as well as variables within the inner function can be mutated which should be taken into account
  • Before parsing the function body we need to apply the mutation algorithm which changes all the mutated variables names in the tokens
• The inner function can mutate the closured variables using <<-
• The inner function can have parameters with the same name as the parent function
• When the parent function calls the inner function
• Keep in mind that inner functions can have functions themselves and so on, need to make sure all the above points apply to them as well.