Looking for champion - std.lang.d.lex

[Sean Kelly]

Sean Kelly <sean at invisibleduck.org> wrote:
> Andrei Alexandrescu <SeeWebsiteForEmail at erdani.org> wrote:
>> On 10/22/10 16:28 CDT, Sean Kelly wrote:
>>> Andrei Alexandrescu Wrote:
>>>> 
>>>> I have in mind the entire implementation of a simple design, but
>>>> never
>>>> had the time to execute on it. The tokenizer would work like this:
>>>> 
>>>> alias Lexer!(
>>>>       "+", "PLUS",
>>>>       "-", "MINUS",
>>>>       "+=", "PLUS_EQ",
>>>>       ...
>>>>       "if", "IF",
>>>>       "else", "ELSE"
>>>>       ...
>>>> ) DLexer;
>>>> 
>>>> Such a declaration generates numeric values DLexer.PLUS etc. and
>>>> generates an efficient code that extracts a stream of tokens from a
>>>> stream of text. Each token in the token stream has the ID and the
>>>> text.
>>> 
>>> What about, say, floating-point literals?  It seems like the first
>>> element of a pair might have to be a regex pattern.
>> 
>> 
>> Yah, with regard to such regular patterns (strings, comments,
> > numbers,
>> identifiers) there are at least two possibilities that I see:
>> 
>> 1. Go the full route of allowing regexen in the definition. This is
>> very hard because you need to generate an efficient (N|D)FA during
>> compilation.
>> 
>> 2. Pragmatically allow "fallthrough" routines, i.e. if nothing in the
>> compile-time table matches, just call onUnrecognizedString(). In
>> conjunction with a few simple specialized functions, that makes it
>> very simple to define arbitrarily complex lexers where the bulk of
> > the
>> work (and the most tedious part) is done by the D compiler.
> 
> For the second, that may push the work of recognizing some lexical
> elements into the parser. For example, a comment may be defined as
> /**/,
> which if there is no lexical definition of a comment means that it
> parses as four distinct valid tokens, div mul mul div.

Or maybe not. A /* could be CommentBegin. I'll have to think on it a bit
more.