On 3/14/07, ian <fist_187@softhome.net> wrote:
> Say I have a function called "solveHardProblem".   solveHardProblem relies on
> several helper functions, which are not going to be useful to any other
> functions in the program.  So, my first instinct would be to define all the
> helpers using let blocks within the definition of solveHardProblem.

I usually do exactly that.  I think it's fine to have a long let like
this, so long as it's made up of small, manageable pieces.  The
biggest problem is that your arguments are now miles away from the
definition of your function, but this can be solved with something
like the following:

let solve_hard_problem =
    let solve_simpler_problem x = ...
    and solve_something_else x = ... in
    fun x y z ->
        [code goes here]

This is also a good trick for moving a global variable (such as a
unique ID counter or a hash table) into the scope of the function that
uses it.

FWIW I've seen some examples of this style in the standard library.
genlex.ml has a 150+ line function of the form:

let make_lexer keywords =
  let kwd_table = Hashtbl.create 17 in
  List.iter (fun s -> Hashtbl.add kwd_table s (Kwd s)) keywords;
  let ident_or_keyword id = ...
  and keyword_or_error c = ...
  [snip]
  in
  fun input -> Stream.from (fun count -> next_token input)

Since the "keywords" argument is needed by the helper functions, it
appears at the top in the let, but "input" isn't and so it was moved
to the bottom in the fun.  None of the helper functions are much
longer than one screenful so it makes for easy reading.