List Iterators

The list iterators are based upon the well-known functional programming construct fold, with left-associative and right-associative varieties provided. Since PLAN does not (currently) support language-level parametric polymorphism or higher-order functions the list iterators are provided as language primitives, rather than as proper functions.

• 
  Although the type above is correct, current PLAN programs may only have a function name appearing in the function argument to Foldl (which is only natural, since PLAN does not have anonymous functions). The meaning of

  

  is

  

• 
  As for Foldl, the function argument to Foldr must be a function name. The meaning of

  

  is