Treetop Email Parser

Discussion in 'Ruby' started by Phrogz, Jan 28, 2008.

  1. Phrogz

    Phrogz Guest

    (I would post this to the treetop mailing list...except there doesn't
    seem to be one.)

    Warming up to treetop, at lunch today I ported the official RFC2822
    email address specification to treetop. (As a grammar that allows
    recursive comments, this is notoriously impossible to match exactly
    correctly using regular expressions.)

    C:\>irb
    irb(main):001:0> require 'treetop'
    irb(main):002:0> require 'email_address'
    irb(main):003:0> @e = EmailAddressParser.new
    irb(main):004:0> @e.parse( 'foo' )
    => nil
    irb(main):005:0> @e.parse( '' ).pieces
    => ["foo", "bar.com"]
    irb(main):006:0> @e.parse( '!@[69.46.18.236]' ).pieces
    => ["!", "[69.46.18.236]"]
    irb(main):007:0> @e.parse( '"Gavin Kistner"@[69.46.18.236]' ).pieces
    => ["\"Gavin Kistner\"", "[69.46.18.236]"]

    The only 'cheat' I did was to omit any of the branches of the grammar
    that were marked as obsolete (and yet included?) in the spec.

    I'm not 100% sure that this is working perfectly, however. From the
    ABNF spec:
    addr-spec = local-part "@" domain
    domain = dot-atom / domain-literal / obs-domain
    domain-literal = [CFWS] "[" *([FWS] dcontent) [FWS] "]" [CFWS]
    CFWS = *([FWS] comment) (([FWS] comment) / FWS)
    comment = "(" *([FWS] ccontent) [FWS] ")"
    ccontent = ctext / quoted-pair / comment
    ctext = NO-WS-CTL / ; Non white space controls
    %d33-39 / ; The rest of the US-ASCII
    %d42-91 / ; characters not including "(",
    %d93-126 ; ")", or "\"

    From the above, I would think that the following was a legal email
    address:
    irb(main):008:0> @e.parse( 'foo@[0.0.0.0](ok)' )
    => nil

    As shown, however, Treetop fails to parse it. Did I fail to port ABNF
    to Treetop properly? Here are the relevant pieces:
    rule addr_spec
    local_part "@" domain
    end
    rule domain
    dot_atom / domain_literal
    end
    rule domain_literal
    CFWS? "[" (FWS? dcontent)* FWS? "]" CFWS?
    end
    rule CFWS
    (FWS? comment)* ((FWS? comment) / FWS)
    end
    rule comment
    "(" ( FWS? ccontent )* FWS? ")"
    end
    rule ccontent
    ctext / quoted_pair / comment
    end
    rule ctext
    NO_WS_CTL / [\x21-\x27\x2a-\x5b\x5d-\x7e]
    end

    Following is the full RFC2282 Treetop grammar, in case anyone wants to
    play with it.

    # http://tools.ietf.org/html/rfc2822
    # All obsolete rules have been removed
    grammar EmailAddress

    rule addr_spec
    local_part "@" domain {
    def pieces
    [ local_part.text_value, domain.text_value ]
    end
    }
    end

    rule local_part
    dot_atom / quoted_string
    end

    rule domain
    dot_atom / domain_literal
    end

    rule domain_literal
    CFWS? "[" (FWS? dcontent)* FWS? "]" CFWS?
    end

    rule dcontent
    dtext / quoted_pair
    end

    rule dtext
    NO_WS_CTL / # Non white space controls
    [\x21-\x5a\x5e-\x7e] # The rest of the US-ASCII characters
    # not including "[", "]", or "\"
    end

    # Non-whitespace control characters
    rule NO_WS_CTL
    [\x01-\x08\x0b-\x0c\x0e-\x1f\x7f]
    end

    rule dot_atom
    CFWS? dot_atom_text CFWS?
    end

    rule dot_atom_text
    atext+ ( "." atext+ )*
    end

    # folding white space
    rule FWS
    (WSP* CRLF)? WSP+
    end

    rule CFWS
    (FWS? comment)* ((FWS? comment) / FWS)
    end

    rule CRLF
    "\r\n"
    end

    rule WSP
    [ \t]
    end
    # Any character except controls, SP, and specials.
    rule atext
    ALPHA / DIGIT / [!#$\%&'*+\/=?^_`{|}~-]
    end

    rule ALPHA
    [A-Za-z]
    end

    rule DIGIT
    [0-9]
    end

    rule text
    [\x01-\x09\x0b-\x0c\x0e-\x7f]
    end

    rule specials
    [()<>\[\]:;@\\,.] / DQUOTE
    end

    rule DQUOTE
    '"'
    end

    rule ccontent
    ctext / quoted_pair / comment
    end

    rule quoted_pair
    "\\" text
    end

    rule qtext
    NO_WS_CTL / # Non white space controls
    [0x21\x23-\x5b\x5d-\x7e] # The rest of the US-ASCII characters
    # not including "\" or the quote
    character
    end

    rule qcontent
    qtext / quoted_pair
    end

    rule quoted_string
    CFWS? DQUOTE (FWS? qcontent)* FWS? DQUOTE CFWS?
    end

    rule comment
    "(" ( FWS? ccontent )* FWS? ")"
    end

    rule ctext
    NO_WS_CTL / # Non white space controls
    [\x21-\x27\x2a-\x5b\x5d-\x7e] # The rest of the US-ASCII
    characters
    # not including "(", ")", or "\"
    end
    end
     
    Phrogz, Jan 28, 2008
    #1
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  2. Phrogz wrote:
    > (I would post this to the treetop mailing list...except there doesn't
    > seem to be one.)


    It's fine here. A lot of folk here could benefit from using Treetop,
    if only they could work out how to. Discussion here can help them.

    Your problem lies in the direct translation of this rule, which is
    poorly written in the RFC:

    > CFWS = *([FWS] comment) (([FWS] comment) / FWS)


    to:

    > rule CFWS
    > (FWS? comment)* ((FWS? comment) / FWS)
    > end


    The issue is that the first parenthesized group is a node that
    will eat too much, succeed, then cause the second group to fail.
    This is a feature of PEG parsers in general: once a node has
    succeeded, it will never be retried even though there may be a
    different way it can succeed. Any backtracking that comes to the
    same point will remember that it succeeded here once before, and
    will assume that the *same* success is correct.

    You need to rewrite it to avoid this incorrect success. This rule
    is equivalent:

    rule CFWS
    ( FWS / comment )+
    end

    Just a bit simpler, eh?

    Clifford Heath.

    P.S. It'd be nice to know whether my other Treetop messages were useful to you...?
     
    Clifford Heath, Jan 28, 2008
    #2
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  3. Phrogz

    Phrogz Guest

    On Jan 28, 4:57 pm, Clifford Heath <> wrote:
    > Phrogz wrote:
    > > (I would post this to the treetop mailing list...except there doesn't
    > > seem to be one.)

    >
    > It's fine here. A lot of folk here could benefit from using Treetop,
    > if only they could work out how to. Discussion here can help them.


    Sure. I'd also like a place to send bug reports, however. So far, I've
    been sending them to the only contact address I can find for the
    entire project - Nathan's email as listed in the gemspec.

    > Your problem lies in the direct translation of this rule, which is
    > poorly written in the RFC:
    >
    > >   CFWS            =  *([FWS] comment) (([FWS] comment)/ FWS)

    >
    > to:
    >
    > >   rule CFWS
    > >     (FWS? comment)* ((FWS? comment) / FWS)
    > >   end

    >
    > The issue is that the first parenthesized group is a node that
    > will eat too much, succeed, then cause the second group to fail.
    > This is a feature of PEG parsers in general: once a node has
    > succeeded, it will never be retried even though there may be a
    > different way it can succeed. Any backtracking that comes to the
    > same point will remember that it succeeded here once before, and
    > will assume that the *same* success is correct.
    >
    > You need to rewrite it to avoid this incorrect success. This rule
    > is equivalent:
    >
    > rule CFWS
    >     ( FWS / comment )+
    > end
    >
    > Just a bit simpler, eh?


    You know, I looked at that rule (and some like it) and said "Man, what
    were they smoking!?" But yeah, this was a quick translation over lunch
    and I explicitly didn't want to try translating anything.

    To be fair, that's not quite the same rule, right? Your rule would
    allow FWS FWS FWS, while the original requires a comment between them.
    (FWS itself only eats one CRLF at a time, so this change allows blank
    lines where they were not allowed before. I think.)

    I appreciate the help and insight. This insight scares me, I must
    admit.

    I was happy to accept the no-left-recursion limitation of the grammar,
    because it's easy to spot and the downside is infinite recursion. If
    it wasn't obvious when writing the grammar, it would be reasonably
    obvious at runtime when your parse never finished.

    But what you describe here seems far more insidious. This limitation
    (which I assume is not in fact limited to PEG in general, but
    specifically to the packrat memoization technique that makes this
    solution perform reasonably) says to me: There are some cases where
    you can write something that looks correct and technically *is*
    correct, but that will silently fail on otherwise valid content.
    Please (definitely) correct me if I'm wrong. I definitely don't want
    to spread FUD.

    Can you provide any guidelines or reading points on how to recognize a
    set of rules that may fail like this? Is it limited to use of the zero-
    or-more star symbol? Particularly likely to occur when any star or
    plus quantifier appears at the start of a rule?

    Thanks again for your help. I'm still excited about treetop, but less
    so as I find that I actually have to know some theory about the
    language I'm writing in (*gasp*) instead of being able to naively
    throw together BNF-like rules and have it magically all work out. ;)
     
    Phrogz, Jan 29, 2008
    #3
  4. Phrogz

    Phrogz Guest

    On Jan 28, 8:41 pm, Phrogz <> wrote:
    > On Jan 28, 4:57 pm, Clifford Heath <> wrote:
    > > This is a feature of PEG parsers in general: once a node has
    > > succeeded, it will never be retried even though there may be a
    > > different way it can succeed. Any backtracking that comes to the
    > > same point will remember that it succeeded here once before, and
    > > will assume that the *same* success is correct.

    ....
    > ... This limitation
    > (which I assume is not in fact limited to PEG in general, but
    > specifically to the packrat memoization technique that makes this
    > solution perform reasonably) ...


    My apologies for second guessing you. Having read the PEG paper[1], I
    see now what you're saying. This PEG rule:
    [aeiou]+ 'e'
    behaves quite differently from this regular expression:
    /[aeiou]+e/

    I had thought - based on the use of the terms 'greedy' and
    'backtracking' - that the above PEG rule would be able to match a
    string like "aaae", just like the regexp. I see now that PEG in
    general (packrat or not) are really, REALLY greedy. The above PEG rule
    doesn't match that string because [aeiou]+ consumes the whole string,
    and when it fails to find an 'e', it does NOT backtrack on the number
    of repetitions of the character class.

    Wow.

    So...how would you write a PEG rule to match "a string of any number
    of vowels, that ends with an 'e'"?

    [1] http://pdos.csail.mit.edu/~baford/packrat/popl04/peg-popl04.pdf
     
    Phrogz, Jan 29, 2008
    #4
  5. Phrogz wrote:
    > On Jan 28, 4:57 pm, Clifford Heath <> wrote:
    >> This rule is equivalent:
    >> rule CFWS
    >> ( FWS / comment )+
    >> end
    >> Just a bit simpler, eh?

    > You know, I looked at that rule (and some like it) and said "Man, what
    > were they smoking!?" But yeah, this was a quick translation over lunch
    > and I explicitly didn't want to try translating anything.
    >
    > To be fair, that's not quite the same rule, right? Your rule would
    > allow FWS FWS FWS, while the original requires a comment between them.


    Well, yes-ish. If you look at the definition of FWS, it matches any non-zero
    amount of whitespace, as long as it has only one CRLF (I missed this 2nd
    restriction when checking equivalence). The easiest way to fix this is to
    separate the CRLF from other whitespace, and ensure that there is comment
    text (potentially containing whitespace) on every line except the first and
    last, or something like that. An exercise for the reader ;-).

    > I was happy to accept the no-left-recursion limitation of the grammar,
    > because it's easy to spot and the downside is infinite recursion.


    Yes, that's not something that's actually happened to me, so I've never
    even needed to spot it :). I ran into it big-time when I first used
    TXL though!

    > But what you describe here seems far more insidious. This limitation
    > (which I assume is not in fact limited to PEG in general, but
    > specifically to the packrat memoization technique that makes this
    > solution perform reasonably)


    Correct, I meant the packrat technique, not PEG.

    > says to me: There are some cases where
    > you can write something that looks correct and technically *is*
    > correct, but that will silently fail on otherwise valid content.


    There's a reason why Nathan points out that among a set of alternates,
    the *first* one that succeeds is taken. That means, even if that makes a
    higher-level rule fail, it won't try subsequent alternatives. Yes, it's
    a trap, but once you get into the right thought pattern, it's not that
    hard to avoid. You basically need to consider where you might mis-read
    the text yourself, and introduce negative lookahead that prevents the
    mis-reading.

    Oh, and use TDD :). You need that to ensure that whitespace is
    skipped, or expected, everywhere it should, if for no other reason.
    You just won't get it right with TT otherwise... but you won't with
    any other tool either.

    > Can you provide any guidelines or reading points on how to recognize a
    > set of rules that may fail like this? Is it limited to use of the zero-
    > or-more star symbol? Particularly likely to occur when any star or
    > plus quantifier appears at the start of a rule?


    Not just at the start of a rule, but anywhere in the rule. You need to
    ensure that you insert a !wrong_path predicate to limit the repetition,
    any time a repetition might chew too much. Likewise for alternation,
    make sure you get the alternates in the right order, and dress any with
    negative lookahead when an early choice might be wrong.

    > Thanks again for your help. I'm still excited about treetop, but less
    > so as I find that I actually have to know some theory about the
    > language I'm writing in (*gasp*) instead of being able to naively
    > throw together BNF-like rules and have it magically all work out. ;)


    Have a play with ANTLRworks, which will analyze things and tell you when
    you have a problem, and when you find that ANTLR's other restrictions
    begin to frustrate the crap out of you (as happened to me), come back to
    using Treetop. :)

    Does anyone else wish for the ability of Treetop to emit languages
    other than Ruby? I'm probably going to need C# one day, and perhaps
    Java.

    Clifford Heath.
     
    Clifford Heath, Jan 29, 2008
    #5
  6. Phrogz wrote:
    > On Jan 28, 8:41 pm, Phrogz <> wrote:
    > My apologies for second guessing you. Having read the PEG paper[1], I
    > see now what you're saying.


    Well, though the authors of the packrat paper described a parser that
    behaves that way, I don't think it's fair to limit the possible other
    behaviours of any PEG parser generator. IOW, PEG >= packrat. ANTLR is
    also LL(*), but avoids this problem by analyzing the expressions and
    either rejecting or re-writing them.

    > So...how would you write a PEG rule to match "a string of any number
    > of vowels, that ends with an 'e'"?


    Assuming some EOF token (not provided free by Treetop) you'd write:

    (!('e' EOF) [aeiou])* 'e'

    Of course, in many cases, EOF would be replaced by a white-space or
    other rule.

    Clifford Heath.
     
    Clifford Heath, Jan 29, 2008
    #6
  7. On Jan 28, 10:22 pm, Phrogz <> wrote:
    > The only 'cheat' I did was to omit any of the branches of the grammar
    > that were marked as obsolete (and yet included?) in the spec.


    The "obsolete" productions were not obsolete in RFC 822, and hence
    there may still be
    a significant number of clients out there _capable_ of emitting e-mail
    addresses using
    that syntax.

    However, in practice, most clients generate a very limited subset of
    RFC 822 and/or
    RFC 2822 compliant addresses - it's likely safe to ignore the obsolete
    rules. 8-9
    years ago, when I used to run a webmail system with 1-2 million
    accounts, we didn't
    even bother trying to parse the full RFC as we never saw e-mail coming
    in using the more
    "exotic" features, such as nested comments or quoted local parts.

    Vidar
     
    Vidar Hokstad, Jan 29, 2008
    #7
  8. Phrogz

    Matt Mower Guest

    On 1/29/08, Clifford Heath <> wrote:
    > Does anyone else wish for the ability of Treetop to emit languages
    > other than Ruby? I'm probably going to need C# one day, and perhaps
    > Java.


    Would I be alone in finding Objective-C support useful?

    Regards,

    Matt.

    --
    Matt Mower :: http://matt.blogs.it/
     
    Matt Mower, Jan 29, 2008
    #8
  9. Phrogz

    Phil Tomson Guest

    On 1/29/08, Matt Mower <> wrote:
    > On 1/29/08, Clifford Heath <> wrote:
    > > Does anyone else wish for the ability of Treetop to emit languages
    > > other than Ruby? I'm probably going to need C# one day, and perhaps
    > > Java.

    >
    > Would I be alone in finding Objective-C support useful?
    >


    An Objective-C backend for TreeTop would rock!

    That said, looking at the output from tt it seems that it's very much
    oriented towards Ruby's dynamicity - extend is used extensively, for
    example.

    Not saying an Objective-C backend would be impossible, though....

    Phil
     
    Phil Tomson, Jan 29, 2008
    #9
  10. Phil Tomson wrote:
    > On 1/29/08, Matt Mower <> wrote:
    >> Would I be alone in finding Objective-C support useful?

    > An Objective-C backend for TreeTop would rock!


    Once the basic framework for multiple output languages was there,
    it wouldn't be hard to add. At the moment there's a Ruby builder,
    but the interface doesn't allow a simple substitution of another
    builder. A fairly simple cleanup would fix that. I'd also like it
    to emit more meaningful names, like "rulename_result" and
    "rulename_sequence" instead of "r1" and "s0".

    > That said, looking at the output from tt it seems that it's very much
    > oriented towards Ruby's dynamicity - extend is used extensively, for
    > example.
    > Not saying an Objective-C backend would be impossible, though....


    Not at all. The code blocks would have to be emitted into new classes
    for Java. For C#, they'd be partial classes. I don't know Obj-C, but
    I doubt it's harder than those two.

    Clifford Heath.
     
    Clifford Heath, Jan 29, 2008
    #10
  11. Phrogz

    Matt Mower Guest

    On 1/29/08, Clifford Heath <> wrote:
    > Not at all. The code blocks would have to be emitted into new classes
    > for Java. For C#, they'd be partial classes. I don't know Obj-C, but
    > I doubt it's harder than those two.
    >


    No, probably easier.

    Once basic support for other languages is there I would be glad to get
    involved in generating the Obj-C code.

    I already have a use in mind for Treetop + Objective-C + Nu.

    M.

    --
    Matt Mower :: http://matt.blogs.it/
     
    Matt Mower, Jan 29, 2008
    #11
  12. Phrogz

    Phil Tomson Guest

    On 1/29/08, Clifford Heath <> wrote:
    > Phil Tomson wrote:
    > > On 1/29/08, Matt Mower <> wrote:
    > >> Would I be alone in finding Objective-C support useful?

    > > An Objective-C backend for TreeTop would rock!

    >
    > Once the basic framework for multiple output languages was there,
    > it wouldn't be hard to add. At the moment there's a Ruby builder,
    > but the interface doesn't allow a simple substitution of another
    > builder. A fairly simple cleanup would fix that.


    What do you think would be involved to create this framework for
    multiple output langauages? Any design docs?

    > I'd also like it
    > to emit more meaningful names, like "rulename_result" and
    > "rulename_sequence" instead of "r1" and "s0".



    >
    > > That said, looking at the output from tt it seems that it's very much
    > > oriented towards Ruby's dynamicity - extend is used extensively, for
    > > example.
    > > Not saying an Objective-C backend would be impossible, though....

    >
    > Not at all. The code blocks would have to be emitted into new classes
    > for Java. For C#, they'd be partial classes. I don't know Obj-C, but
    > I doubt it's harder than those two.


    Yes, it's probalby easier in Obj-C.


    Phil
     
    Phil Tomson, Jan 30, 2008
    #12
  13. Phil Tomson wrote:
    > What do you think would be involved to create this framework for
    > multiple output langauages? Any design docs?


    No design docs, but the Ruby text is built using a Ruby builder,
    which makes it sound fine and ready to go... except that the API
    to the builder allows arbitrary text strings to be appended, and
    the API assumes modules and extend() etc... Basically the builder
    API needs to be cleaned up to encapsulate the semantics and to not
    assume Ruby. Then when doing the first *other* language, cleaned
    up some more to catch the things you missed the first time. :)

    Clifford Heath.
     
    Clifford Heath, Jan 31, 2008
    #13
  14. Phrogz

    Phil Tomson Guest

    On 1/30/08, Clifford Heath <> wrote:
    > Phil Tomson wrote:
    > > What do you think would be involved to create this framework for
    > > multiple output langauages? Any design docs?

    >
    > No design docs, but the Ruby text is built using a Ruby builder,
    > which makes it sound fine and ready to go... except that the API
    > to the builder allows arbitrary text strings to be appended, and
    > the API assumes modules and extend() etc... Basically the builder
    > API needs to be cleaned up to encapsulate the semantics and to not
    > assume Ruby. Then when doing the first *other* language, cleaned
    > up some more to catch the things you missed the first time. :)
    >


    Yeah, I noticed the Builder stuff. I did some changes
    (experimentally) to my local version of TreeTop to add an
    on_success_callback that gets called when a rule parses
    successfully... turns out I didn't need that functionality just now,
    but it did get me familiar with some of the inards.

    Thinking out loud here... So let's aay somebody did a C++ backend. As
    has been mentioned, the current builder makes use of Ruby's extend
    functionality quite heavily - modules get built for each alternative
    and then get mixed in as needed. To get the functionality in C++ I
    suppose you could just build new classes for each rule alternative.

    For example the following snippet from tt:

    module TopLevelSav1
    def get_ports
    puts "unknown ports"
    end
    def port_decl?
    false
    end
    def entity_decl?
    puts "Not an entity decl!"
    false
    end
    end

    def _nt_top_level_sav
    start_index = index
    cached = node_cache[:top_level_sav][index]
    if cached
    @index = cached.interval.end
    return cached
    end

    i0, nr0 = index, []
    r1 = _nt_entity_decl
    r1.extend(TopLevelSav0)
    nr0 << r1
    if r1.success?
    r0 = r1
    r1.update_nested_results(nr0)
    else
    s2, nr2, i2 = [], [], index
    loop do
    r3 = parse_anything(SyntaxNode)
    nr2 << r3
    if r3.success?
    s2 << r3
    else
    break
    end
    end
    r2 = AnyNode.new(input, i2...index, s2, nr2)
    r2.extend(TopLevelSav1)
    ....

    The second extend there seems fairly straighforward: when you generate
    the AnyNode class, generate a subclass that includes the methods
    defined in TopLevelSav1. The first extend seems a bit more
    problematic. We get a SyntaxNode object back from the
    _nt_entity_decl. We could define a class TopLevelSav0 that includes
    the methods defined in that module and then pass the SyntaxNode in and
    construct it like so:

    class TopLevelSav1 : public SyntaxNode {
    public:
    vector< Port* > getports();
    bool port_decl();
    bool entity_decl();
    TopLevelSav1(SyntaxNode* sn);
    }

    ... later:

    SytaxNode* r1;
    ...
    r1 = _nt_entity_decl();
    r1 = new TopLevelSav1(r1);

    Or maybe better yet:

    r1 = dynamic_cast<TopLevelSav1*>(_nt_entity_decl());

    Thoughts?

    Oh, BTW: right now we allow Ruby code in rules:

    rule foo
    'something' {
    def got_something?
    true
    end
    }
    end

    How would we go about this for other languages?

    rule foo
    'something' {
    #C++ code:
    bool got_something() { return true; } // but how would you
    handle scoping here?
    }
    end

    It seems to me that I'd like to prototype the grammar in Ruby and then
    transfer it over to C++ (or ObjC ) later on after I'm sure everything
    is working. How could we make this multi-language targetting easier?

    Phil
     
    Phil Tomson, Feb 1, 2008
    #14
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