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Alan Syntax Notes

Some working notes on how to scope the Alan syntax elements.

Table of Contents

Notes on Identifiers

A valid identifier in Alan has the following pattern:

IDENTIFIER : LETTER ( LETTER | DIGIT | '_' )*

... where LETTER means:

  • any char in the Ascii range a..z
  • any char in the Ascii range A..Z
  • any char in the Unicode range U+00E0..U+00F6 (à..ö)
  • any char in the Unicode range U+00F8..U+00FE (ø..þ)

The two Unicode ranges represent characters from the Latin-1 Supplement (ISO-8859-1), which is the default encoding expected by Alan.

Here is the full ISO-8859-1 subset of valid characters in an identifier:

0 1 2 3 4 5 6 7 8 9
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
_
a b c d e f g h i j k l m n o p q r s t u v w x y z
à á â ã ä å æ ç è é ê ë ì í î ï ð ñ ò ó ô õ ö ø ù ú û ü ý þ

RegEx Aproach

To simplify, the Unicode range can be described as the set U+00E0..U+00FE minus U+00F7 (obelus symbol: ÷).

The valid letters can be represented via the following RegExs, in order of simplification:

  1. [a-zA-Z\x{00E0}-\x{00F6}\x{00F8}-\x{00FE}]
  2. [a-zA-Z\x{00E0}-\x{00FE}&&[^\x{00F7}]]
  3. [a-zA-Zà-þ&&[^÷]]

The full identifier pattern is: (\b['a-zA-Zà-þ&&[^÷]][0-9_'a-zA-Zà-þ&&[^÷]]*\b), which in the sytnax source is represent through variables:

variables:
  LETTER: 'a-zA-Zà-þ&&[^÷]'
  ID: '(\b[{{LETTER}}][0-9_{{LETTER}}]*\b)'

Known Issues

While the above RegEx does a fairly good job at matching valid identifiers and ignoring malformed ones, the presence of invalid symbols characters (including ÷) in the middle of a word will cause the syntax parser to split the word — because most symbols satisfy the boundry condition of \b. Therefore, an invalid identifer like abc÷123 will be actually parsed as:

  • abc — identifier
  • ÷ — stray symbol
  • 123 — numeric constant

There isn't much I can do about it; besides, we don't expect users to employ symbols in identifiers — if they chose to do so, at their own peril!

Alan Sources BNF References

The BNF definion of Identifier in "alan.g" source (line 15):

fragment IDENTIFIER : LETTER ( LETTER | DIGIT | '_' )* ;

Also, in "alan.g" (line 9):

fragment LETTER : 'A' .. 'Z' | 'a' .. 'z' | '_' | '$' | '\u00e0'..'\u00f6' | '\u00f8'..'\u00fe' ;

NOTE — The latter BNF definition might lead to assume that _ and $ are also valid characters in the LETTER range, but it is not so (these don't reflect the actual validations carried out by the Alan compiler in real use). The $ symbol is never a valid character in identifiers, and the _ can't be used as the first character of an identifier.

Syntax Scopes

A few notes about Alan language syntax elements and their scoping.

Classes

Class scopes:

  • entity.name.class
  • entity.name.class.forward-decl
  • entity.other.inherited-class

Class Attributes

Fields, properties, members and attributes of a class or other data structure should use:

variable.other.member

Pre-Defined Alan Classes

There are eight pre-defined (hard-coded) Alan classes:

  • entity
    • thing
      • object
      • actor *
    • location *
    • literal
      • string
      • integer

NOTE 1 — actor and location are found in the Alan Keywords list, but not the other hard-coded classes.

NOTE 2 — hero doesn't appear in the Alan Keywords list either, although it's a hard-coded instance.

Special Keywords

THIS should be scoped as variable.language (for reserved language variables like this, super, self, etc.)

Alan Keywords

There are 123 keywords in Alan 3 language, as mentioned in the Reference Manual (Appendix D.2 Keywords).
actor add after an and are
article at attributes before between by
can cancel character characters check container
contains count current decrease definite depend
depending describe description directly do does
each else elsif empty end entered
event every exclude exit extract first
for form from has header here
if import in include increase indefinite
initialize into is isa it last
limits list locate location look make
max mentioned message min name near
nearby negative no not of off
on only opaque option options or
play prompt pronoun quit random restart
restore save say schedule score script
set show start step stop strip
style sum synonyms syntax system taking
the then this to transcript until
use verb visits wait when where
with word words

Reference Links