D port of dmd: Lexer, Parser, AND CodeGenerator fully operational

[Jonathan M Davis]

On Friday, March 09, 2012 00:54:48 Dmitry Olshansky wrote:
> On 08.03.2012 22:46, Jonathan M Davis wrote:
> > On Thursday, March 08, 2012 22:03:12 Dmitry Olshansky wrote:
> >> On 08.03.2012 11:48, Jonathan M Davis wrote:
> >>> A range is not necessarily a dynamic array, though a dynamic array is a
> >>> range. The lexer is going to need to take a range of dchar (which may or
> >>> may not be an array), and it's probably going to need to return a range
> >>> of tokens. The parser would then take a range of tokens and then output
> >>> the AST in some form or other - it probably couldn't be range, but I'm
> >>> not sure. And while the lexer would need to operate on generic ranges of
> >>> dchar, it would probably have to be special-cased for strings in a
> >>> number
> >>> of places in order to make it faster (e.g. checking the first char in a
> >>> string rather than using front when it's known that the value being
> >>> checked against is an ASCII character and will therefore fit in a single
> >>> char - front has to decode the next character, which is less efficient).
> >> 
> >> Simply put, the decisison on decoding should belong to lexer. Thus
> >> strings should be wrapped as input range of char, wchar& dchar
> >> respectively.
> > 
> > ??? The normal way to handle this is to simply special-case certain
> > operations. e.g.
> > 
> > static if(Unqual!(isElementEncodingType!R) == char)
> > { ... }
> 
> Does isElementEncodingType aware of anything other then string/wstring?

No. It uses isNarrowString. So, what you'd end up doing is having the lexer 
accept a generic range of dchar and then have specializations where 
appropriate for narrow strings. Nothing in Phobos' string handling really 
supports the idea of dealing with generic char or wchar ranges. It all 
processes ranges of dchar and specializes on narrow strings where appropriate.

But is there really a use case for a generic range of char or wchar? I don't 
know. In general, I really don't think that there is. When dealing with ranges 
of characters, they're essentially always either strings or strings which have 
been wrapped in other ranges (generally by calling some other range-based 
function such as take or filter). And those range-based functions pretty much 
inevitably need to treat the strings as ranges of dchar to do what they do 
(potentially with specific optimizations for strings). They aren't designed to 
operate on ranges of char or wchar, and the only reason to make them do so 
would be if there were a use case where you needed a range of char or wchar 
which was not an array. But they're all either arrays or wrapped arrays. So, 
no such use case currently exists with Phobos, and I really question the 
usefulness of trying to optimize on a generic range of char or wchar - 
especially when many of the optimizations for arrays involve random access 
ranges, and if you have a random access range of char or wchar, I _really_ 
question that it would ever be anything other than an array.

So, I'd advise against trying to operate on ranges of char or wchar and just 
stick to operating on ranges of dchar with optimizations for narrow strings 
where appropriate.

> Now, speaking outside of this specific problem.
> Basically I would propose formalizing a kind of range that current
> string/wstring is. And that is a VariableLengthEncoding range (VLE
> range), a two in one - random access codeunit range and bidirectional
> 'codepoint' range. I've heard of attempts on this concept before, but
> now with a use case at hand it could be become more important.

There has been some talk of VLE ranges but really only with regards to the 
fact that strings are a case of that. Nothing has been done to generalize it. 
It may be something that should be looked into, but until we've formalized 
that, I don't think that something like the lexer should be built around the 
idea. It would be safer to stick with what we've been doing - operating on 
ranges of dchar and special-casing for narrow strings where appropriate. If 
need be, it should be possible to adjust it to use VLEs later.

> The problem is, I think, that current InputRange range is insufficent as
> it requres to calculate length of first element twice: one time in front
> and extra one in popFront.

Perhaps an optional function should be added to input ranges which both 
returns and front and pops it. But VLE ranges such as narrow strings are 
probably the only ones which would really care, and we can already special 
case for strings, so it would probably only be of real value for general VLEs. 
However, since VLEs would arguably be a new range type, they could just 
implement the new function, and anything which special-cases VLEs can take 
advantage of it, while they still work just fine as input ranges. We could even 
add a free function which both pops and returns front which uses front and 
popFront for most ranges and the new function for VLEs, so you can code with 
that free function in cases where you intend to both take front and pop it off 
without caring about what type of range you're dealing with and without 
forcing ranges in general to implement an extra function.

There's definitely more work and designing to be done here, but there are 
definitely possibilities.

Still, in the interim, I'd advise simply special casing on narrow strings in 
the lexer rather than trying to use VLEs initially.

- Jonathan M Davis