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From: Gleki Arxokuna <gleki.is.my.name@gmail.com>
Date: Tue, 7 Apr 2015 11:28:12 +0300
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Subject: [bpfk] One cannot refer to inner nodes in Lojban PEG
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Consider the following:
string_1 = string_2 / string_6 string_3
string_3 = string_7 / string_8
string_4 = !string_1 string_5

Now we need !string_1 to work only if string_8 matches, not when string_7
matches.
Why we might need that? E.g. for terms that can have either FA or BAI
inside. Depending on whether we have FA or BAI inside a term our actions
may differ.

How can we implement that?
We can just create duplicates of two strings like this:

string_1 = string_2 / string_6 string_3
string_1_2 = string_2 / string_6 string_3_2
string_3 = string_7 / string_8
string_3_2 = string_8
string_4 = !string_1_2 string_5

Now string_1_2 is the same as string_1 but it is matched only if string_8
matches (unlike string_1, otherwise they are the same).

Now the question is how do we avoid such duplication?

The answer from peg.js developers:
<quote>
Sorry, but there is currently no clean way to avoid the duplication.

In the future I'm thinking about adding support for rule
templates/parametrizable rules, which would help here:

string_1<T> = string_2 / string_6 T
string_3 = string_7 / string_8
string_3_2 = string_8
string_4 = !string_1<string_3_2> string_5

And I think it'd be nice
</quote>

So for now our only chance is to copy paster parts of Lojban PEG if we want
such things. I.e. one would need to split terms into "terms_for_BAI",
"terms_for_FA" etc. with all their inner variables copied.

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<div dir=3D"ltr"><div>Consider the following:</div><div>string_1 =3D string=
_2 / string_6 string_3</div><div>string_3 =3D string_7 / string_8</div><div=
>string_4 =3D !string_1 string_5=C2=A0</div><div><br></div><div>Now we need=
 !string_1 to work only if string_8 matches, not when string_7 matches.</di=
v><div>Why we might need that? E.g. for terms that can have either FA or BA=
I inside. Depending on whether we have FA or BAI inside a term our actions =
may differ.</div><div><br></div><div>How can we implement that?</div><div>W=
e can just create duplicates of two strings like this:</div><div><br></div>=
<div>string_1 =3D string_2 / string_6 string_3</div><div>string_1_2 =3D str=
ing_2 / string_6 string_3_2</div><div>string_3 =3D string_7 / string_8</div=
><div>string_3_2 =3D string_8</div><div>string_4 =3D !string_1_2 string_5=
=C2=A0</div><div><br></div><div>Now string_1_2 is the same as string_1 but =
it is matched only if string_8 matches (unlike string_1, otherwise they are=
 the same).</div><div><br></div><div>Now the question is how do we avoid su=
ch duplication?</div><div><br></div><div>The answer from peg.js developers:=
</div><div>&lt;quote&gt;</div><div>Sorry, but there is currently no clean w=
ay to avoid the duplication.</div><div><br></div><div>In the future I&#39;m=
 thinking about adding support for rule templates/parametrizable rules, whi=
ch would help here:</div><div><br></div><div>string_1&lt;T&gt; =3D string_2=
 / string_6 T</div><div>string_3 =3D string_7 / string_8</div><div>string_3=
_2 =3D string_8</div><div>string_4 =3D !string_1&lt;string_3_2&gt; string_5=
=C2=A0</div><div><br></div><div>And I think it&#39;d be nice=C2=A0</div><di=
v>&lt;/quote&gt;<br></div><div><br></div><div>So for now our only chance is=
 to copy paster parts of Lojban PEG if we want such things. I.e. one would =
need to split terms into &quot;terms_for_BAI&quot;, &quot;terms_for_FA&quot=
; etc. with all their inner variables copied.</div></div>

<p></p>

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