[erlang-questions] A proposal for Unicode variable and atom names in Erlang.
Fri Oct 19 17:10:27 CEST 2012
I have converted the text below into EEP 40 and enlisted it.
It would be nice if Richard could check it and see if anything
got lost in translation. Especially the odd line "Trouble Spot"
is not supposed to be there, I guess, but I kept it.
/ Raimo Niskanen, as EEP editor
On Fri, Oct 19, 2012 at 07:06:43PM +1300, Richard O'Keefe wrote:
> If it were still possible to submit EEPs in plain text,
> this would be an EEP. If someone else would like to
> package this up as an EEP and submit it (under their
> name, mine, or both), feel free.
> (1) Support for Unicode continues to increase, with
> minimal source code support about to arrive.
> (2) Unicode variable names and unquoted atoms are not
> here yet, so now is the time to settle on a design.
> (3) They will need to come. There may be legal or
> institutional reasons why unicode-capable languages
> are required. Some people just want to use their
> own language and script. Erlang's strength in
> network applications means that being able to
> represent Internationalized Domain Names as unquoted
> atoms would be just as much of a convenience as
> being able to represent ASCII domain names like
> www.example.com (which needs no quotes in Erlang) is.
> (4) There is a framework for Unicode identifiers in
> Unicode standard annex 31 (UAX#31), and several
> programming languages, including Ada, Java,
> and see also http://www.python.org/dev/peps/pep-3131/
> (5) Existing Erlang identifiers should remain valid,
> including ones containing "@" and ".".
> (6) Existing Erlang support features, such as ignoring
> names of the form [_][a-zA-Z0-9_]* when reporting
> singleton variables, should not be broken.
> (7) We should not "steal" any characters to use as "magic
> markers" for variables because they might be needed for
> other purposes. A good (bad) example of this is "?", which
> could be used for several things if it were not used for macros.
> Names of sets of characters, XID_Start, XID_Continue, Lu, Lt, Lo, Pc,
> Other_Id_Start, are drawn from Unicode and UAX#31.
> Lu = upper case letters
> Lt = title case letters
> Pc = connector punctuators, including the low line (_) and
> a number of other characters like undertie (‿).
> Other_Id_Start = script capital p, estimated symbol,
> katakana-hiragana voiced sound mark, and
> katakana-hiragana semi-voiced sound mark.
> variable ::= var_start var_continue*
> var_start ::= XID_Start ∩ (Lu ∪ Lt ∪ Pc ∪ Other_Id_Start)
> var_continue ::= XID_Continue U "@"
> The choice of XID here follows Python. It ensures that the normalisation
> of a variable is still a variable. In fact Unicode variables should be
> normalised. Unicode has enough look-alike characters that we cannot hope
> for "look the same <=> are the same" to be true, but we should go _some_
> way in that direction.
> Variables in scripts that do not distinguish letter case have to
> begin with _some_ special character to ensure that they are not
> mistaken for unquoted atoms. There are 10 Pc characters in the Basic
> Multilingual Plane. The Erlang parser treats a variable beginning
> with an underscore specially: there will be no complaint if it is a
> singleton. There are 9 other Pc characters for which this special
> treatment is not applied. Of course, someone might be using fonts
> that do include say Arabic letters but not say the undertie. We can
> deal with that by revising the underscore rule.
> Variable does not begin with a Pc character =>
> should not be a singleton.
> Variable is just a Pc character and nothing else =>
> is a wild card.
> Variable begins with a Pc character followed by a
> Latin-1 character =>
> may be a singleton.
> Variable begins with a Pc character following by
> a character outside the Latin-1 range =>
> should not be a singleton.
> Thus ‿ is a wild-card, 隠者 is an atom, _隠者 should not be
> a singleton, but __隠者 _may_ be a singleton. This rule is a
> consistent generalisation of the existing rule.
> Unquoted atoms
> unquoted_atom ::= atom_start atom_continue
> atom_start ::= XID_Start \ (Lu ∪ Lt ∪ Lo ∪ Pc)
> | "." (Ll ∪ Lo)
> atom_continue ::= XID_Continue U "@"
> | "." (Ll ∪ Lo)
> Again the choice of XID follows Python, and ensures that the
> normalisation of an unquoted atom is still an unquoted atom.
> Unquoted atoms should be normalised.
> The details of Erlang unquoted atoms are somewhat subtle; I have
> checked my understanding experimentally.
> Keywords have the form of unquoted atoms. No new keywords are
> - Any Python identifier or keyword is
> an Erlang variable or unquoted atom or keyword.
> - @ signs may occur freely in variables and unquoted atoms except as the
> first character, as now.
> - dots may not be followed by capital letters, digits, or underscores,
> as now.
> - I am not sure whether modifier letters should be allowed after a dot.
> - I am not sure what to do with the Other_ID_Start characters.
> Script capital p _looks_ like a capital p and even has "capital" in
> its name. All other "* SCRIPT CAPITAL *" characters are upper case
> letters. Surely it should be allowed to start a variable.
> The estimated sign looks like an enlarged lower case e; other symbols
> that look like letters are classified as letters. You'd expect this
> to begin an atom. As for the Katakana-Hiragana voicing marks, I have
> no intuition whatever. Assigning the whole group to atoms seems
> - All existing variable names and unquoted atoms remain legal, and no
> new variable or atom forms using only Latin-1 characters have been
> Trouble spot
> erlang-questions mailing list
/ Raimo Niskanen, Erlang/OTP, Ericsson AB
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