[erlang-questions] Why do we need modules at all?

Gordon Guthrie <>
Tue May 24 10:48:09 CEST 2011

There are two good things about modules:
* they are a higher level of abstraction
* they support good working practices

Lets take a module that I use a little bit - and which a colleague
introduced me to:

I can use the couple of exposed functions of that that I know,
confident that if it comes to it I can come back and master the rest
of the interface later. I have confidence that there is a rubustness
there that makes code using it maintainable. (my idea of maintainable
is to be able to ignore most stuff, most of the time...). I can learn
about the code at this higher level of abstraction.

We too have a module with loads of random functions in it, well we
have three (hn_util, util, util2), and each of them is just library
functions. In the utility modules the ratio of exported to
not-exported fns is maybe 7 to 1.

In functional modules the exported/not ratio goes from 1-1 (in modules
that define apis) to 1-3 against.

If we had the (promised) -export-to() directive the number of pure
exported fns would drop.

Given that we build our testing strategy around -export() this
separation is critical...


On 24 May 2011 09:06, Joe Armstrong <> wrote:
> Why do we need modules at all?
> This is a brain-dump-stream-of-consciousness-thing. I've been
> thinking about this for a while.
> I'm proposing a slightly different way of programming here
> The basic idea is
>     - do away with modules
>     - all functions have unique distinct names
>     - all functions have (lots of) meta data
>     - all functions go into a global (searchable) Key-value database
>     - we need letrec
>     - contribution to open source can be as simple as
>       contributing a single function
>     - there are no "open source projects" - only "the open source
>       Key-Value database of all functions"
>     - Content is peer reviewed
> These are discussed in no particular order below:
> Why does Erlang have modules?
> There's a good an bad side to modules:
> Good: Provides a unit of compilation, a unit of code
> distribution. unit of code replacement
> Bad: It's very difficult to decide which module to put an individual
> function in. Break encapsulation (see later)
> Aside: lib_misc.erl
> When I'm programming I often get to the point were I say there should
> a function foo/2 in lists.erl but their isn't. There should be but
> there isn't - foo/2 is a small self contained thing. Why should it be
> in lists.erl because it "feels right".
> Strings are lists, so why do we have two modules lists.erl and
> string.erl how should I decide in which module my new string/list
> processing function should go.
> To avoid all mental anguish when I need a small function that
> should be somewhere else and isn't I stick it in
> a module elib1_misc.erl.
> My elib1_misc exports the following:
> added_files/2                 make_challenge/0
> as_bits/1                     make_response/2
> as_bits_test/0                make_response_test/0
> bdump/2                       make_test_strings/1
> bin2hex/1                     make_test_strings_test/0
> bin2hex_test/0                make_tmp_filename/2
> check_io_list/1               merge_kv/1
> collect_atom/1                merge_kv_test/0
> collect_atom_test/0           mini_shell/0
> collect_int/1                 module_info/0
> collect_int_test/0            module_info/1
> collect_string/1              ndots/1
> collect_string_test/0         nibble_to_hex_char/1
> collect_word/1                nibble_to_hex_char_test/0
> complete/2                    odd/1
> complete_test/0               on_exit/2
> dos2unix/1                    out_of_date/2
> downcase_char/1               outfile/2
> dump/2                        padd/2
> dump_tmp/2                    perms/1
> duplicates/1                  perms_test/0
> ensure_started/2              pmap/2
> eval_file/1                   pmap1/2
> eval_file_test/0              pmap1_test/0
> eval_string/1                 pmap_test/0
> eval_string_test/0            priority_receive/0
> every/3                       random_seed/0
> expand_env_vars/1             random_string/1
> expand_file_template/3        random_string/2
> expand_string_template/2      read_at_most_n_lines/2
> expand_tabs/1                 read_at_most_n_lines_test/0
> expand_tabs_test/0            remove_duplicates/1
> expand_template/2             remove_duplicates_test/0
> extract_attribute/2           remove_leading_and_trailing_whitespace/1
> extract_attribute_test/0      remove_leading_and_trailing_whitespace_test/0
> extract_prefix/2              remove_leading_whitespace/1
> fetch/2                       remove_prefix/2
> fetch_test/0                  remove_prefix_test/0
> file2lines/1                  remove_trailing_whitespace/1
> file2lines_test/0             replace/3
> file2md5/1                    root_dir/0
> file2numberedlines/1          rpc/2
> file2numberedlines_test/0     safe/1
> file2paras/1                  show_loaded/1
> file2stream/1                 signed_byte_to_hex_string/1
> file2string/1                 signed_byte_to_hex_string_test/0
> file2template/1               skip_blanks/1
> file2term/1                   skip_blanks_test/0
> file_size_and_type/1          skip_to_nl/1
> find_src/1                    skip_to_nl_test/0
> first/1                       sleep/1
> flatten_io_list/1             spawn_monitor/3
> flush_buffer/0                split_at_char/2
> for/3                         split_at_char_test/0
> force/1                       split_list/2
> foreach_chunk_in_file/3       split_list_test/0
> foreach_word_in_file/2        string2exprs/1
> foreach_word_in_string/2      string2exprs_test/0
> forever/0                     string2html/1
> get_erl_section/2             string2latex/1
> get_line/1                    string2lines/1
> get_line/2                    string2lines_test/0
> have_common_prefix/1          string2stream/1
> have_common_prefix_test/0     string2stream_test/0
> hex2bin/1                     string2template/1
> hex2bin_test/0                string2template_test/0
> hex2list/1                    string2term/1
> hex2list_test/0               string2term_test/0
> hex_nibble2int/1              string2toks/1
> hex_nibble2int_test/0         string2toks_test/0
> id/1                          sub_binary/3
> include_dir/0                 template2file/3
> include_file/1                term2file/2
> interleave/2                  term2string/1
> is_alphanum/1                 test/0
> is_blank_line/1               test1_test/0
> is_prefix/2                   test_function_over_substrings/2
> is_prefix_test/0              tex2pdf/1
> is_response_correct/3         time_fun/2
> keep_alive/2                  time_stamp/0
> lines2para/1                  to_lower/1
> list2frequency_distribution/1 to_lower_test/0
> list2frequency_distribution_tetrim/1
> longest_common_prefix/1       trim_test/0
> longest_common_prefix_test/0  unconsult/2
> lookup/2                      unsigned_byte_to_hex_string/1
> lorem/1                       unsigned_byte_to_hex_string_test/0
> ls/1                          which/1
>                               which_added/1
> Now I find this very convenient when I write a new small utility function
> I stick in in elib1_misc.erl - no mental anguish in choosing a module
> name is involved.
> The observation that I find this very-convenient is telling me something
> about modules - I like my elib1_misc it feels right.
> (aside - It seems many development projects have their own private
> lib_miscs ...)
> Which brings me to the point of my question.
> Do we need module's at all? Erlang programs are composed of lots of small
> functions, the only place where modules seem useful is to hide a letrec.
> The classic example is fibonacci. We want to expose fib/1 but hide the
> helper function fib/3. Using modules we say
> -module(math).
> -export([fib/1]).
> fib(N) ->
>     fib(N, 1, 0).
> fib(N, A, B) when N < 2 -> A;
> fib(N, A, B) -> fib(N-1, A+B, A).
> The downside is we have had to *invent* one module name math - whose *only*
> purpose is to hide the definition of fib/3 which we don't want to be made
> callable.
> If we put a second function into the module math, then this second function
> could call fib/3 which breaks the encapsulation of fib/3.
> We could say:
> let fib = fun(N) -> fib(N, 1, 0) end
> in
>    fib(N, A, B) when N < 2 -> A;
>    fib(N, A, B) -> fib(N-1, A+B, A).
> end.
> I hardly dare suggest a syntax for this since I've been following
> another thread in this forum where syntax discussion seem to encourage
> much comment.
> ** Please do suggest alternative syntax's here - but do not comment on
> other peoples suggestions ...
> I would like to just talk about why we have modules.
> Another question:
> Does the idea of a module come from the idea that functions have to be
> stored somewhere, so we store them in a file, and we slurp the
> file (as a unit) into the system, so the file becomes a module?
> If all the files were store by themselves in a database would this
> change things.
> I am thinking more and more that if would be nice to have *all* functions in
> a key_value database with unique names.
> lookup(foo,2) would get the definition foo foo/2 from a database.
> The unique names bit is interesting - is this a good idea. Qualified
> names (ie names like xxx:foo/2) or (a.b.c.foo/2) sounds like a good
> idea but but when I'm programming I have to invent the xxx or the
> a.b.c which is very difficult. It also involves the "decision problem"
> if the namespaces xxx and a.b.c already exist I have to *choose* which
> to put my new function in.
> I think there might be a case for alises here joe:foo/2 could be used
> while developing "joe" would expand to a horrible random local string the
> real name being ab123aZwerasch123123_foo/2  but I would not be able to
> publish my code or make it available to a third_part before I had
> chosen a sensible name.
> (( managing namespaces seems really tricky, a lot of peoople seem
> to thing that the problem goes away by adding "." 's to the name
> but managing a namespace with namees like foo.bar.baz.z is just as complex
> as managing a namespace with names like foo_bar_baz_z or names like
> 0x3af312a78a3f1ae123 - the problem is that we have to go from a symbolic
> name like www.a.b to a reference like - but how do we discover
> the initial name www.a.b? - there are two answers - a) we are given the name
> (ie we click on a link) - we do not know the name but we search fo it ))
> When programs are small we can live with "just the code" in "a few
> modules" the ratio of code to meta data is high.
> When programs are large we need a lot of meta-data to understand them.
> I would like to see all functions with all meta-data in a data base.
> I'd like to say:
>    lookup(foo,2,Attribute) when Attribute =
>       code|source|documentation|type signatures|revision history|authors|...
> The more I think about it the more I think program development should
> viewed as changing the state of a Key-Value database.
> So I imagine:
>     1) all functions have unique names
>     2) there are no modules
>     3) we discover the name of a function by searching metadata
>        describing the function in a database
>     4) all public functions (think open source) are in the same
>        database
> We could make a system to do this.
> I think this would make open-source projects easier, since the
> granularity of contribution goes down. You could contribute
> a single function - not an entire application.
> (( A problem with GUT style open source projects is there is
>    not one database of functions, I often what one function from
>    this project, another function from another project -- the
>    granularity of reusable parts should be the individual function.
>    functions are really easy to reuse
>    modules are more difficult to reuse
>    entire applications are very difficult to reuse
>      (Unless there are isolated through a communication channel))
> Possible extensions.
>     1) Voting for promotion
>     2) A review process
> Given a raw database will *all* functions in it - we could derive an
> "approved" functions database.
> Popular functions could be moved to the approved database - the
> review process would need to be discussed - so kind of peer-review/wiki
> stuff.
> Comments?
> Volunteers?
> /Joe
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Gordon Guthrie
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