I have made available the first preview of a second module for OCaml Batteries Included module: Enum. This module builds upon ExtLib’s enumerations (which it means to replace, if it is accepted upstream) and provides support for representation-independent iterators. These iterators are used pervasively in ExtLib and will also be used pervasively in the rest of Batteries Included, as a manner of converting data from/to data structures and as a base for syntax extensions.

With respect to ExtLib’s current implementation, this release adds

• numerous powerful constructors and manipulation functions
• functions inspired from SDFlow and dedicated to cooperative
• better management of infinite iterators
• better management of iterators created using from
• syntactic sugar.

As an example of the last point, let us note that it is now possible, without any Camlp4 extension, to replace for loops with a more (stream-)functional counterpart. That is, instead of

for i = 1 to n do
    (*...*)
done

one may now write

iter (fun i -> (* ... *) )
  (5 -- 10)

for an imperative loop or

map (fun i -> (* ... *) )
  (5 -- 10)

for a lazy transformation, etc. It won’t improve performance and it doesn’t look more readable at first glance, but it allows short expressions such as :

iter printf (5 -- 10) (*to print all numbers between 5 and 10*)
map ( ~ ) (5 --10) (*to obtain enumeration -5, -6, -7, -8, -9, -10 *)
fold ( + ) 0 ( 5 -- 10 )(*to sum all numbers between 5 and 10*)

etc. Everything is computed lazily, without allocating any intermediate data structure.

Code may be found here.