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From: Gleki Arxokuna <gleki.is.my.name@gmail.com>
Date: Wed, 22 Nov 2017 14:45:27 +0300
Message-ID: <CAO7bV+gt8M3u-d3oCc8BVELy4qjG8ie5B3wmL9iCvYyb+t3VFQ@mail.gmail.com>
Subject: Re: [lojban] Re: CLL and modern Lojban
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2017-11-22 14:38 GMT+03:00 Remo Dentato <rdentato@gmail.com>:

> On Wed, Nov 22, 2017 at 10:29 AM, Gleki Arxokuna <
> gleki.is.my.name@gmail.com> wrote:
>
>> PEG has no tools to check for ambiguity, it simply does not support it,
>> it's another grammar with its own drawbacks. It can't be compared to YACC,
>> it's just another class of grammars.
>>
>> If you want tools resolving ambiguity then having tools not more powerful
>> than PEG or CFG is not enough.
>>
>
>
> Since the absence of syntactic ambiguity is one of (my) requirement for
> Lojban, let me understand better.
>
> In CFG the source of ambiguity is the at any given moment in parsing one
> can choos which alternative path to follow.
> So, for example, the following CFG grammar is ambiguous:
>
> E = E '+' E | E '-' E | N
> N = '0' | '1' | ... | '9'
>
> because the string "4+3-9" may result in two different parse tree:
>
>          E                 E
>        / | \             / | \
>       E  +  E           E  -  E
>       |   / | \       / | \   |
>       4  E  -  E     E  +  E  9
>          |     |     |     |
>          3     9     4     3
>
> The corresponding PEG would be:
>
> E = E '+' E / E '-' E | N
> N = '0' / '1' / ... / '9'
>
> And there is only parse tree for "4+3+9"
>          E
>        / | \
>       E  +  E
>       |   / | \
>       4  E  -  E
>          |     |
>          3     9
>
> (note that PEG do support left recursion even if the original packrat
> algorithm didn't)
>
> CFG grammars need to be checked for ambiguities
>

yes,that.


> and have to be rewritten to eliminate them whereas PEG do not have this
> problem (and hence lack a tool for checking ambiguities).
>

They simply don't support it since they have priority choice operator that
disallows check for ambiguities and requires strict order of subrules.

I already checked PEG grammars proposed by BPFK and discovered that
changing this operator back to alternation operator leads to lots of
ambiguities.

So BPFK simply cheated by saying "only the first AST is correct". Since
people here are also interested in learnability of Lojban by humans this
proposed change of grammar makes it potentially much harder. Who would
remember that the rule for lujvo comes after cmavo (or maybe vice versa?).
CFG doesn't have this problem.

That's only one drawback of PEG in general if we don't delve into specific
proposed grammars.


> I do agree that ensuring that a PEG grammar does exactly what you want it
> to do is not easy (especially if it's a complex grammar) but "per se" the
> PEG itself does not introduce any ambiguity.
>
>
> Is this what you meant or am I missing something?
>
> --
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<div dir=3D"ltr"><br><div class=3D"gmail_extra"><br><div class=3D"gmail_quo=
te">2017-11-22 14:38 GMT+03:00 Remo Dentato <span dir=3D"ltr">&lt;<a href=
=3D"mailto:rdentato@gmail.com" target=3D"_blank">rdentato@gmail.com</a>&gt;=
</span>:<br><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;bo=
rder-left:1px #ccc solid;padding-left:1ex"><div dir=3D"ltr"><div class=3D"g=
mail_extra"><div class=3D"gmail_quote"><span class=3D"">On Wed, Nov 22, 201=
7 at 10:29 AM, Gleki Arxokuna <span dir=3D"ltr">&lt;<a href=3D"mailto:gleki=
.is.my.name@gmail.com" target=3D"_blank">gleki.is.my.name@gmail.com</a>&gt;=
</span> wrote:<br><blockquote class=3D"gmail_quote" style=3D"margin:0px 0px=
 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div di=
r=3D"ltr"><span class=3D"m_-7153330886369609961gmail-"></span><span class=
=3D"m_-7153330886369609961gmail-"></span><div class=3D"gmail_extra"><div cl=
ass=3D"gmail_quote"><div>PEG has no tools to check for ambiguity, it simply=
 does not support it, it&#39;s another grammar with its own drawbacks. It c=
an&#39;t be compared to YACC, it&#39;s just another class of grammars.</div=
><div><br></div><div>If you want tools resolving ambiguity then having tool=
s not more powerful than PEG or CFG is not enough.</div></div></div></div><=
/blockquote><div><br></div><div><br></div></span><div>Since the absence of =
syntactic ambiguity is one of (my) requirement for Lojban, let me understan=
d better.</div><div><br></div><div>In CFG the source of ambiguity is the at=
 any given moment in parsing one can choos which alternative path to follow=
.</div><div> So, for example, the following CFG grammar is ambiguous:</div>=
<div><br></div>E =3D E &#39;+&#39; E | E &#39;-&#39; E | N</div><div class=
=3D"gmail_quote">N =3D &#39;0&#39; | &#39;1&#39; | ... | &#39;9&#39;</div><=
div class=3D"gmail_quote"><br></div><div class=3D"gmail_quote">because the =
string &quot;4+3-9&quot; may result in two different parse tree:</div><div =
class=3D"gmail_quote"><br></div><font face=3D"monospace,monospace">=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 E=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 E<br>=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 / | \=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 / | \<br>=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0 E=C2=A0 +=C2=A0 E=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0 E=C2=A0 -=C2=A0 E<br>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 |=C2=A0=C2=A0 / =
| \=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 / | \=C2=A0=C2=A0 |<br>=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0 4=C2=A0 E=C2=A0 -=C2=A0 E=C2=A0=C2=A0=C2=A0=C2=A0 E=C2=
=A0 +=C2=A0 E=C2=A0 9<br>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 |=
=C2=A0=C2=A0=C2=A0=C2=A0 |=C2=A0=C2=A0=C2=A0=C2=A0 |=C2=A0=C2=A0=C2=A0=C2=
=A0 |<br>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 3=C2=A0=C2=A0=C2=
=A0=C2=A0 9=C2=A0=C2=A0=C2=A0=C2=A0 4=C2=A0=C2=A0=C2=A0=C2=A0 3</font><div =
class=3D"gmail_quote">=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 <br></div><div class=
=3D"gmail_quote"><div>The corresponding PEG would be:</div><div><br></div><=
div>E =3D E &#39;+&#39; E / E &#39;-&#39; E | N<div class=3D"gmail_quote">N=
 =3D &#39;0&#39; / &#39;1&#39; / ... / &#39;9&#39;</div><div class=3D"gmail=
_quote"><br></div><div class=3D"gmail_quote">And there is only parse tree f=
or &quot;4+3+9&quot;<br></div><div class=3D"gmail_quote"><font face=3D"mono=
space,monospace">=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 E=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 <br>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 / |=
 \=C2=A0=C2=A0=C2=A0=C2=A0 <br>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 E=C2=A0 +=C2=
=A0 E=C2=A0=C2=A0=C2=A0 <br>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 |=C2=A0=C2=A0 / =
| \=C2=A0 <br>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 4=C2=A0 E=C2=A0 -=C2=A0 E <br>=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 |=C2=A0=C2=A0=C2=A0=C2=A0 =
| <br>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 3=C2=A0=C2=A0=C2=A0=
=C2=A0 9 <br></font></div><div class=3D"gmail_quote"><br></div><div class=
=3D"gmail_quote">(note that PEG do support left recursion even if the origi=
nal packrat algorithm didn&#39;t)</div><div class=3D"gmail_quote"><br></div=
><div class=3D"gmail_quote">CFG grammars need to be checked for ambiguities=
</div></div></div></div></div></blockquote><div><br></div><div>yes,that.</d=
iv><div>=C2=A0</div><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0=
 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir=3D"ltr"><div cl=
ass=3D"gmail_extra"><div class=3D"gmail_quote"><div><div class=3D"gmail_quo=
te"> and have to be rewritten to eliminate them whereas PEG do not have thi=
s problem (and hence lack a tool for checking ambiguities).</div></div></di=
v></div></div></blockquote><div><br></div><div>They simply don&#39;t suppor=
t it since they have priority choice operator that disallows check for ambi=
guities and requires strict order of subrules.</div><div><br></div><div>I a=
lready checked PEG grammars proposed by BPFK and discovered that changing t=
his operator back to alternation operator leads to lots of ambiguities.</di=
v><div><br></div><div>So BPFK simply cheated by saying &quot;only the first=
 AST is correct&quot;. Since people here are also interested in learnabilit=
y of Lojban by humans this proposed change of grammar makes it potentially =
much harder. Who would remember that the rule for lujvo comes after cmavo (=
or maybe vice versa?). CFG doesn&#39;t have this problem.</div><div><br></d=
iv><div>That&#39;s only one drawback of PEG in general if we don&#39;t delv=
e into specific proposed grammars.</div><div><br></div><div><br></div><bloc=
kquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #cc=
c solid;padding-left:1ex"><div dir=3D"ltr"><div class=3D"gmail_extra"><div =
class=3D"gmail_quote"><div><div class=3D"gmail_quote"><br></div><div class=
=3D"gmail_quote">I do agree that ensuring that a PEG grammar does exactly w=
hat you want it to do is not easy (especially if it&#39;s a complex grammar=
) but &quot;per se&quot; the PEG itself does not introduce any ambiguity.<b=
r></div><div class=3D"gmail_quote"><br></div><div class=3D"gmail_quote"><br=
></div>Is this what you meant or am I missing something? <br></div></div></=
div><div class=3D"gmail_extra"><br></div></div><div class=3D"HOEnZb"><div c=
lass=3D"h5">

<p></p>

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</div></div></blockquote></div><br></div></div>

<p></p>

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