Grammar Troubles: "juxtaposition" operator

[Nick Johnson]

Hello,

I am trying to implement a grammar with an odd feature.  This grammar
must allow for a "juxtaposition" operator--an implicit operator
between two expressions.  When I remove the juxtaposition operator,
the language works exactly as I would expect.  However, once I add the
juxtaposition operator, it exhibits unexpected behavior.

The grammar is listed below in its barest terms.  Here, the last rule
having the lowest precedence:

expr -> 'a'
expr -> '(' expr ')'
expr -> '~' expr
expr -> expr '*'
expr -> expr expr       // juxtaposition operator
expr -> expr '|' expr

I have observed at least two problems:

(1) The juxtaposition operator appears to be right-associative, though
it is declared left-associative; and

(2) The juxtaposition operator has a higher precedence than the '~'
operator, though the opposite is declared.



I do not think this is a Bison bug; rather, I think it is a bug in my
grammar.  I think the absense of a terminal token in the juxtaposition
rule causes things to mess up.  If anyone could help me out, I would
really appreciate it.


I have attached a minimal yacc file (minimal.y) which translates
simple expressions (such as testcase.in) into a graphical parse-tree
(using graphviz).  To build and run this, perform:

$ bison minimal.y
$ gcc minimal.tab.c -o minimal
$ ./minimal <testcase.in | dot -Tjpg >testcase.jpg

I have also attached a sample output (testcase.jpg) to save you the
trouble.  I am using Bison version 2.3, built from source, and gcc
version 3.4.4 on linux.

Thank you,
-- 
Nick Johnson

minimal.y
Description: Binary data

testcase.in
Description: Binary data

testcase.jpg
Description: JPEG image