<aside> 💡 The Reference Rule states that whenever the type comment of a self-referential data type references another non-primitive data type, a natural helper (processing the other data type) should be written within a function template of the self-referential data type.

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• Whenever the type comment of a self-referential data type references another data type, a “natural helper” — a function for the referenced type — should be written within a function template of the self-referential data type.

Data definitions which are subject to the reference rule The data definition for ListOfSchool references School. As a result, the function template for fn-for-los (a function for ListOfSchool) will feature fn-for-school (a function for School) as a natural helper.

• Natural helpers in the templates are the precursors for actual helper functions.

• When designing function templates with natural helpers within them,

  • delegate some of the complicated work within the self-referential function to the helper function (e.g. for a function producing a bar chart, the helper function can help to produce 1 bar)
  • create a wish-list entry for the actual helper function

<aside> 🧠 When you ultimately get to designing the function template for the natural helper, take reference from the check-expect examples written in the self-referential function template. In fact, you might be able to copy some important details from the examples to the helper function definition (but certainly not the other way round).

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• Here’s an example of helper functions in action.