Recommendations (top-down)

The correctness and types of the program are all determined bottom-up as described previously, but inner expressions sometimes need to be nudged in the right direction. Consider the following example:

(et-defvar my-list List<Cons<String_Positive>>
  (cons (cons "Alice" 1.5) nil))

(cons "Alice" 1.5) evaluates to the type ConsFresh<"Alice"_1.5>; however, as described in Fresh types, this must be resolved to a concrete cons type before proceeding. Of the many options it has to resolve to, we can clearly see that it should choose Cons<String_Positive>, but it needs some way to know that. To make this possible, a type-recommendation must be passed down when traversing an expression.
et-defvar type-checks the top-level cons with the recommendation List<Cons<String_Positive>>. Based on this, cons calculates that it should give its first child the recommendation Cons<String_Positive>. Then, when (cons "Alice" 1.5) is converted to a concrete type, it uses the recommendation as a hint, and returns the correct type.