[erlang-questions] Why do we need modules at all?
Ralf P. Gans
Ralf.Gans@REDACTED
Tue May 24 10:11:32 CEST 2011
Hi,
the ability to replace code in a running environment depends, to my knowledge, on modules.
So modules prevent downtimes and allow 7/24 operation.
cheers, Ralf
On 24.05.2011, at 10:06, 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 123.45.23.12 - 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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