Re: Examples of disambiguation

On Tuesday 07 July 2026 00:38:11 (+02:00), Bethan Tovey-Walsh wrote:


Thanks for sharing this, Steven. 


What are the changes to the grammar that describe this construct? And what exactly are its semantics? (From what you've described, I'm assuming that it's a kind of negative lookahead.)


       -term: factor;
               option;
               prohibition; {added}
               repeat0;
               repeat1.
       prohibition: factor, -"!", s.


The semantics are:


 In an alternative such as 
 
  A, B!, C


 if B succeeds (successfully parses) at that position, B! fails, and so the alternative fails; if B fails, B! succeeds, and C is started at the position B was started.


Steven






BTW



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Dr. Bethan Tovey-Walsh

linguacelta.com <http://linguacelta.com/>

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On 6 Jul 2026, at 16:22, Steven Pemberton <steven.pemberton@cwi.nl> wrote:


In the new implementation, we decided to use the notation "x!".


The reasoning behind this is that all similar constructs in ixml are also postfix:


 x* zero or more
 x+ one or more
 x? zero or one
 x! zero


and prefix characters are use for marks on serialisation.


So here are some example disambiguation grammars.


IDENTIFIERS AND KEYWORDS
Each line is either an identifier or a keyword


  input: ((id; keyword), #a)*.
     id: keyword!, letter+, letter!.
-letter: [L].
keyword: ("if"; "then"; "else"), letter!.


Input
i
if
ifi
the
then
thene


Output
<input>
  <id>i</id>
  <keyword>if</keyword>
  <id>ifi</id>
  <id>the</id>
  <keyword>then</keyword>
  <id>thene</id>
</input>


MIXED CHARACTERS
Taken from the ixml tutorial, separating input characters into classes
{example input: 
...abcd()1234!!!}
  input: line+.
   line: (number; word; punc)*, 
-#a.
 number: digit+, digit!.
   word: letter+, letter!.
   punc: 
p+, p!.
-letter: [L].
 -digit: ["0"-"9"].
     -p: 
[P].
Output
<input>
  
<line>
    <punc>...</punc>
    
<word>abcd</word>
    
<punc>()</punc>
    
<number>1234</number>
    
<punc>!!!</punc>
  
</line>
</input>
CODES
Each line is either a header that ends with a code, or an item that is a series of words.
catalogue: entry*.
    entry: header, 
item+.
   header: text, -" ", code, -#a.
    
 item: (text, " ")?, code!, word, -#a.
    -text: word**" 
".
    -word: (l; d)+, (l;d)!.
    @code: l, l, 
l, d, d, d.
       -l: [L].
      
 -d: ["0"-"9"].
Input
Fiction 
fic001
Brave New World
1984
NonFiction non123
Translating 
Beaudelaire
The Sixth 
Extinction


Output
<catalogue>
  
<entry>
    <header 
code="fic001">Fiction</header>
    <item>Brave 
New World</item>
    
<item>1984</item>
  </entry>
  
<entry>
    <header 
code="non123">NonFiction</header>
    
<item>Translating Beaudelaire</item>
    
<item>The Sixth Extinction</item>
  
</entry>
</catalogue>
Steven