[erlang-questions] Which is best? string:concat or ++?

[Richard O'Keefe]

On 8/05/2012, at 3:39 AM, Paul Barry wrote:
> 
>    http://dev.af83.com/2012/01/16/erlang-iolist.html

Under the heading IOList, the third paragraph gets list
concatenation about as wrong as it possibly can.
(The last sentence of that paragraph is right, though.)

Let's consider a simple "nested list" representation of
strings.

nstring = integer | [] | [nstring|nstring]
  integer represents a codepoint
  [] represents an empty string
  [A|B] represents (but is not) the concatenation of A and B

% Convert an nstring to a plain string.
% Do not allocate any space that is not part of the
% final result.

reify_nstring(NS) ->
   reify_nstring(NS, []).

reify_nstring([A|B], S) ->
    reify_nstring(A, reify_nstring(B, S));
reify_nstring([], S) ->
    S;
reify_nstring(C, S) when is_integer(C), C >= 0 ->
    [C|S].

% Perform F(C) for each character C in nstring S.

foreach_nstring(F, [A|B]) ->
    foreach_nstring(F, A),
    foreach_nstring(F, B);
foreach_nstring(F, []) ->
    ok;
foreach_nstring(F, C) when is_integer(C), C >= 0 ->
    F(C).

% Report the effective length of an nstring in codepoints.

length_nstring(S) ->
    length_nstring(S, 0).

length_nstring([A|B], N) ->
    length_nstring(B, length_nstring(A, N));
length_nstring([], N) ->
    N;
length_nstring(C, N) when is_integer(C), C >= 0 ->
    N + 1.

You can generalise this to allow atoms and binaries
as well as character codes.

The basic idea is to *describe* concatenation without
*doing* it, and to give you functions that act *as if*
the concatenation had been done.