On Thursday, 23 January 2014 at 08:12:08 UTC, OP wrote:
I'd like Walter to reply to this. In his article here
http://www.drdobbs.com/architecture-and-design/so-you-want-to-write-your-own-language/240165488
Walter says
Somewhat more controversial, I wouldn't bother wasting time
with lexer or parser generators and other so-called "compiler
compilers." They're a waste of time. Writing a lexer and
parser is a tiny percentage of the job of writing a compiler.
Using a generator will take up about as much time as writing
one by hand, and it will marry you to the generator (which
matters when porting the compiler to a new platform). And
generators also have the unfortunate reputation of emitting
lousy error messages.
I have no idea how to write one. First off when I originally
tried before using anything I wrote nonsense code. Second is I
couldn't tell if I had a conflict (either shift reduce or
reduce reduce) or if I wrote it wrong/badly. Third is I'm not
sure how a tiny one should look like and my language is fairly
large. I already understand flex and bison. I'm planning to
rewrite from scratch so I could attempt doing so on my own but
I don't know how to deal with the above.
I just looked at parse.c and I had no idea there is a
precedence table. Why is there one rather than it being
embedded like a switch statement which tries to handle all the
higher precedence operations calling a function running the
next set of precedence? I guess that does sound like a
loop/table but I imagined it in switch statements.
Using bison I can write complex statements which are easy for
me to grok and change. I wouldn't know how to change complex
statements if I hand wrote the parser. I don't know all the
places something like that would affect. Taking the syntax
below I'm not sure how to fork a state and discard the invalid
one.
foo[5] = var2
foo[5] foo = var2
Here when I see foo[5] I'm either accessing an array (first
statement) or declaring variables as an array of 5 elements
(second statement). Just like `Foo&foo` could be a reference or
could be an AND statement. Foo is definitely processed on its
own, I don't know how to process it as both (a fork) and
continue on the parser to find a valid path.
My bison file has >2000 states. I'm not sure how much work it
may mean if I handwrote my parser.
Hand written parsers are usually LL(K), you just need to write
the grammar in a way that is LL(K) compliant.
Granted there are a few cases where only a LALR(k) parser will do
(yacc and friends), but most context free grammars tend to be
LL(K) as well.
There are lots of tutorials how to write such types of compilers,
for example,
http://compilers.iecc.com/crenshaw/
http://www.ethoberon.ethz.ch/WirthPubl/CBEAll.pdf
http://llvm.org/docs/tutorial/
--
Paulo
