Re: reverse a list. is this a good solution

Martin DeMello [email protected]
[ocaml_beginners] schreef op 20-10-2014 21:42:

Let's unpack it a bit. Going back to your
original function:

let rec test l1
l2 =

match l1 with

| [] -> l2

| h :: t ->
test t (h::l2)

;;

this works, but it has the
problem that l2 should always be an empty list for the
initial call. One way to solve it is to add that to the
function documentation:

(* NOTE: Always call this with l2 = [] *)

let rec test l1
l2 =

match l1 with

| [] -> l2

| h :: t ->
test t (h::l2)

;;

but that
is pretty fragile; the user could always ignore
that. A better way is to provide a second, wrapper
function that takes only one argument, l1, and then
calls your original function with l1 and []:

let rec test
l1 l2 =

match l1 with

| [] -> l2

| h :: t
-> test t (h::l2)

;;

let
reverse_list l1 =

test l1 []

This is a
lot better already - you cannot go wrong if you call
reverse_list. But there is still the issue that test
is exposed, and could inadvertently be called. If
you think about it, you want a function to reverse a
list, and therefore it should only take one
argument, l1. So where did l2 come from? The issue
is that if we are building up a list via recursive
calls to the same function, we need a second
argument, the accumulator, to store the in-progress
intermediate results. But this is not really part of
your function interface; it is an implementation
detail that has been exposed to the user. What you
*want* is for reverse_list to be your public
interface, and test to be hidden entirely. The
common way to do that is to define test entirely
within the body of reverse_list:

let reverse_list l1 =

(* here we define the 'test'
function *)

let rec
test l1 l2 =

match
l1 with

| []
-> l2

| h
:: t -> test t (h::l2)

in

(* here
'test' has been defined and is available *)

test l1 []

(*
here 'reverse_list' has been defined and is
available but

'test' is no longer in scope

*)

now the
user can only call reverse_list with a single list
argument, l1, which is exactly what you want.

martin

Thanks all.

Now I try to make this idea work with a palingdrome. But I think I
need two seperate functions

Roelof

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Posted by: Roelof Wobben <[email protected]>
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