On 8/1/2012 8:04 PM, Jonathan M Davis wrote:
On Wednesday, August 01, 2012 17:10:07 Walter Bright wrote:
1. It should accept as input an input range of UTF8. I feel it is a mistake
to templatize it for UTF16 and UTF32. Anyone desiring to feed it UTF16
should use an 'adapter' range to convert the input to UTF8. (This is what
component programming is all about.)
But that's not how ranges of characters work. They're ranges of dchar. Ranges
don't operate on UTF-8 or UTF-16. They operate on UTF-32. You'd have to create
special wrappers around string or wstring to have ranges of UTF-8. The way
that it's normally done is to have ranges of dchar and then special-case
range-based functions for strings. Then the function can operate on any range
of dchar but still operates on strings efficiently.
I have argued against making ranges of characters dchars, because of performance
reasons. This will especially adversely affect the performance of the lexer.
The fact is, files are normally in UTF8 and just about everything else is in
UTF8. Prematurely converting to UTF-32 is a performance disaster. Note that the
single largest thing holding back regex performance is that premature conversion
to dchar and back to char.
If lexer is required to accept dchar ranges, its performance will drop at least
in half, and people are going to go reinvent their own lexers.
So, it's quite possible to make a lexer operate on ranges of dchar but be
operating primarily on ASCII by special-casing strings and avoiding decoding
as much as possible. My lexer does a good job of this, I believe, so it's
thoroughly optimized for strings, but it's still operating on ranges of dchar,
not ranges of UTF-8.
2. It should output an input range of tokens
3. tokens should be values, not classes
4. It should avoid memory allocation as much as possible
5. It should read or write any mutable global state outside of its "Lexer"
instance
6. A single "Lexer" instance should be able to serially accept input ranges,
sharing and updating one identifier table
When you say identifier table, do you mean symbol table, or something else?
All identifiers are entered into a hashtable, and are referred to by pointers
into that hashtable for the rest of dmd. This makes symbol lookups incredibly
fast, as no string comparisons are done.
Isn't the symbol table something for the parser to worry about? Other than
parsers, I can't think of anything which would even _care_ about a symbol
table. And depending on how a symbol table were done, you'd want it to take
scoping rules into account (_I'd_ certainly expect it to), meaning that it
only includes identifiers which are relevant to the current scope. And if
that's the case, it _really_ doesn't belong in the lexer. The code using the
lexer can easily have its own table that it populates according to however it
wants it populated by processing the identifier tokens as they get lexed. Not
to mention, don't symbol tables usually include type information that a lexer
_wouldn't_ have?
The symbol table becomes a symbol table of pointers into the lexer's identifier
table.
Sure, a symbol table _could_ be put in the lexer (at least if it doesn't care
about scoping), but I definitely question that that's the right place for it.
The symbol table isn't in the lexer, the identifier string table is.
If you mean a table that simply lists identifiers rather than a symbol table,
I'm not quite sure why you'd want it in addition to the symbol table, and
again, I'd expect only parsers to care about that sort of thing, so I'd expect
the parser to be implementing it.
Because it is gawdammed fast to do it that way. An identifier is treated as a
string exactly once, in the lexer, and thereafter by its unique handle.
So, what exactly are you looking for the lexer to implement as far as an
identifier table goes, and why is it in the lexer rather than the parser?
Very simple. A mapping of [identifier string] => [unique value], and a pointer
servers nicely as that unique value.
7. It should accept a callback delegate for errors. That delegate should
decide whether to:
1. ignore the error (and "Lexer" will try to recover and continue)
2. print an error message (and "Lexer" will try to recover and continue)
3. throw an exception, "Lexer" is done with that input range
I'm currently treating errors as tokens. It then becomes easy for the code
using the lexer to just ignore the errors, to process them immediately, or to
put off dealing with them until the lexing is complete. So, the code using the
lexer can handle errors however and whenever it likes without having to worry
about delegates or exceptions. Since tokens are lexed lazily, the fact that an
error is reported as a token doesn't require that the lexing continue, but it
also makes it _easy_ to continue lexing, ignoring or saving the error. I'm
inclined to think that that's a superior approach to using delegates and
exceptions.
I don't agree. It means that the parser has to check everywhere for error
tokens. Besides the ugliness, it means a slowdown for those checks.
8. Lexer should be configurable as to whether it should collect information
about comments and ddoc comments or not
9. Comments and ddoc comments should be attached to the next following
token, they should not themselves be tokens
Why?
Because then the parser would have to add checks for 0 or more comment tokens
between every other token. It uglifies the parser pretty badly, and slows it down.
I'd argue for just processing them as tokens just like I'm doing with
errors. The code using the lexer can then process them or ignore them as it
likes (it can even specifically look for them and ignore all other tokens if
it's something which only operates on comments). And if you want to see which
token follows the comment, it's the next one in the range. So, I don't see
much need to attach comments to other tokens. What's the advantage of not
treating them as tokens?
Performance and simplicity. Your grammar will be awfully ugly unless you insert
another range to filter out those tokens - but that'll be a slowdown.
12. It should come with unittests that, using -cov, show 100% coverage
100% is actually unreasonable, because there are lines which should _never_ be
hit (e.g. an assert(0) line in a switch statement). All lines _other_ than
those sort of lines should have code coverage, but depending on the lexer
implementation, 100% is impossible. std.datetime has a ton of unit tests and
IIRC it still only manages 98% because of assert(0) lines. So, I agree with
the spirit of your statement, but it's not necessarily reasonable to do
exactly what you're saying.
I know, but I expect all unreachable code to be justified. The lexer is a well
defined problem, and this should not be difficult.
