1. Brian Mearns
  2. templ

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templ / syntax

For clarity, the following uses a variation of the EBNF. Specifically, instead of using curly braces '{ symbol }' to indicate 0 or more occurrences, we will use a star 'symbol*' for this purpose, akin to common use in regular expressions. Additionally, we will simply use whitespace instead of command ',' to indicate concatenation, and we will not use the termination symbol ';' at the end of an equation. To continue equations across multiple lines, all subsequent lines will start with whitespace. The equation ends when a line starts with non-whitspace, or contains only whitespace. You can probably figure this all out for yourself.

TEMPLATE        =   STMT+
STMT            =   AWLGT | ESTMT
AWLGT           =   ">>>"                                       (* You may be familiar with this symbol as the "a whole lot
                                                                    greater than" symbol. *)
ESTMT            =   TEXT | LIST
TEXT            =   TCHAR TCHAR*
TCHAR           =   PTCHAR | '\{' | '\' PTCHAR                  (* The next several rules define what can appear in the ordinary
                        | '\' EOI | RANGLES                         text portion of the template. Special characters at this point
PTCHAR          =   ( CHAR - XTCHAR )                               in the parsing are the slash, the left curly brace, and the
XTCHAR          =   '\' | '{' | '>'                                 right angle bracket. Left curly brace can be included if it's
                                                                    escaped by a slash. Slash can only appear on it's own. The right
                                                                    angle bracket character can appear in single or double, but not
                                                                    in triple. *)

EOI             =   ? end of input stream ?
CHAR            =   ? any character ?
LIST            =   '{' WIZ* '}'
WIZ             =   (WS WS*) | EXPR | COMMENT                   (* Lists are made of whitespace, expressions, and comments. *)
WS              =   ? any whitespace character ?
EXPR            =   SYMBOL | LIST | EMBEDTEMPLATE
SYMBOL          =   NAME+
NAME            =   SCHAR | LITERAL
SCHAR           =   ( CHAR - XSCHAR ) | ANGLES | '\'            (* symbols can't include whitespace, and can't used the reserved
                                                                    characters " or % (used for string literals and comments,
                                                                    respectively), and can't include the list delimiters { or }.
                                                                    They also can't use triple angle brackets. *)

XSCHAR          =   WS | XTCHAR | '"' | '%' | '<' | '>'         (* Characters tha can't be used in symbols (more or less). *)
ANGLES          =   LANGLES | RANGLES
LANGLES         =   ('<' XLANGLE) | ('<<' XLANGLE)              (* A single or double left angle-bracket, but not a triple. *)
RANGLES         =   ('>' XRANGLE) | ('>>' XRANGLE)              (* as LANGLES, but with right angle brackets instead. *)
XLANGLE         =   ( SCHAR - '<' )                             (* anything other than the left tangle brackter character. *)
XRANGLE         =   ( SCHAR - '>' )                             (* anything other than the right tangle brackter character. *)
LITERAL         =   STRLITERAL                                  (* there is only one type of literal, a string literal. *)
STRLITERAL      =   '"' DQCHAR* '"'                             (* A string literal is a double quoted string. Note that there is no
                                                                    single quoted string. *)
DQCHAR          =   ( CHAR - XDQCHAR ) | '\' CHAR               (* double-quoted string can contain anything except a slash and a double
                                                                    quote char, or absolutely anything if it's preceeded with a slash. *)

XDQCHAR         =   '"' | '\'                                   (* Characters that can't be included in a double-quoted string. *)
EMBEDTEMPLATE   =   '<<<' ETEMPLATE '>>>'                       (* An embedded template is almost exactly like
ETEMPLATE       =   ESTMT*                                          a top level template, except that it can't contain three
                                                                    right angle brackets in a row. *)

COMMENT         =   '%' LCHAR*                                  (* Comments begin with % and end with the first end of line (or end of input) *)
LCHAR           =   ( CHAR - EOL )                              (* A "line" character, meaning a character that doesn't end a line.*)
EOL             =   ? any end of line character sequence ?

Processing occurs one STMT at a time. Procesing of each STMT is done in two phases: parsing and evaluation. For TEXT elements, the parser collects the longest TEXT it can get. For AWLGT and LIST elements, there is only one possible length of element (i.e., they are unambiguosly terminated). In this case, the parser simply collects one such element.

LIST elements describe the construction of a List object. A List object is an ordered sequence of values (symbols, literals, and other lists). Whitespace (WS) and COMMENTS are not included in the List object, they are essentially ignored by the parser except that they are used as punctuators to separate the different EXPRs in the LIST. In constructing this List object, the parser does not do any evaluation of the elements within the LIST, it simply builds up a List object, possibly containing nested List objects, as described by the LIST syntax.

Embedded templates (EMBEDTEMPLATE) are syntactic sugar to make it easier to produce very large Strings. When an embedded template is encountered during parsing, it is parsed into it's components ESTMTs until the terminating AWLGT (">>>") is encoutnered. This parsing is done *without* evaluation of the ESTMTs, unlike the top level parser which evaluates each STMT as it is parsed. List objects are still built up from all LIST statements encountered in the embedded template just like the parser does at the top level, but they are not evaluated, and TEXT elements are not written to the output stream. Instead, all of the ESTMTs in the embedded template are packed into a List object, in the order in which they appear, and the parser provides this List object as the value of the ESTMT. In other words, the corresponding List object takes the place of the ETEMPLATE in the parent LIST. Note that since resolution of ETEMPLATEs is doen by the parser, nested ETEMPLATEs are likewise resolved to List objects are part of this process, but still are not evaluated.

For evaluation, top level TEXT and AWLGT elements are simply written directly to the output stream. For LIST elements, the LIST is evaluated as an EXPR. If the results of this evaluation is a String, then it is written to the output stream. If it is a Null object, evaluation is done and nothing is written to the output stream. Otherwise, it is an error: top level expressions must evaluate to Strings or Nulls.

For evaluation of EXPRs, SYMBOLs are "self-evaluating", meaning they evaluate to a String object equal to the textual content of the SYMBOL. Notice that a LITERAL is not an alternative to a SYMBOL, it is part of a SYMBOL. In otherwords, literals and non-literals can co-exist in a symbol, for instance FOO"BAR"BAZ. Literals and non literals are just two different ways of describing strings, with literals being used to include characters that non-literals cannot include (for instance, a double-quote character, a curly brace character, or whitespace characters). Thus, the symbol FOO"BAR"BAZ evaluated to the String FOOBARBAZ, of length 9.

On a light note, it would probably be more appropriate to refer to non-LITERAL SYMBOLs as literal, because they describe literally the resulting symbol, where as actual LITERALs are interpretted to produce the resulting symbol. But it would probably confuse a lot of verteran programmers if a quoted string was not the thing referred to as a "literal".

LIST type EXPRs are evaluated as follows. If there are no element in the list, then it evaluates to a Null value. Otherwise, the first element in the list is evaluated (as an EXPR) to produce the TAG.

The TAG is then _resolved_ to an executable. If the TAG is already an Executable, then it is self-resolving, meaning the result of resolving is simply itself. If the TAG is a String, then it is resolved by looking it up in the stack. If a String TAG does not exist in the stack, it is an error. If the value associated with a String TAG in the stack is not an Executable, it is an error. If the TAG is neither a String nor an Executable, it is an error.

Once the TAG is resolved, it is _executed_, using all subsequent elements of the list as the arguments to the executable.

There are three types of executables: functions, operators, and macros. Functions are the simplest: all arguments are first evaluated in full, in order from left to right (lowest to highest index). The results of these evaluations are then used as the arguments passed into the function, in the corresponding order. This is what you would typically expect from other programming languages.

Operators and macros are special: in both cases the arguments are NOT evaluated before being passed to the executable. Thus, even though an element of the List may itself be a List which would normally be evaluated as an expression (i.e., if it was the argument for a function), for an operator or macro, the argument is the list itself. Operators are built into the processor and can evaluate individual arguments on their own. This is necessary for simple implementation of familiar control structures like loops and conditionals. Operators evaluate to values just like functions do.

Like operators, macros receive their arguments unevaluated. However, unlike macros, operators do not evaluate their arguments on their own (in general [1]). In fact, macros may be defined outside of the processor, including in the TEMPLATE itself, so they have no ability to evaluate their arguments. Instead, macros return their own expressions, which the processor then evaluates in the normal way. The result of this subsequent evaluation is the result of the macro.

[1] - Some macros are defined for convenience in the processor and can trigger evaluation. This is done for performance reasons: the macros could just as well be done in the conventional way without evaluating their arguments, it just might take a little longer.

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