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<  Erlang  ~  Precedence handling in yecc of Parse Tools 1.4.5
Bill M.
Posted: Wed Mar 18, 2009 2:03 pm Reply with quote
User Joined: 06 Jun 2008 Posts: 24 Location: New York
Hello All:

Sorry if you get this twice, but to elicit a response, I am cross posting here from the mailing lists. I am using parse tools 1.4.5 on Erlang R12B4.

I was trying out leex and yecc, and tried to do the canonical infix expression parsing, but some strange software behaviors have me wondering if I made a cockpit error or if I've hit some bugs. The source code for the parser appears quoted at the bottom of this e-mail, as well as the build instructions I used.


1). Precedence handling in yecc did not have the desired results, using the grammar with precedence enabled gives the following errors.

Quote:

erl
Erlang (BEAM) emulator version 5.6.4 [source] [64-bit] [smp:2] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.6.4 (abort with ^G)
1> prefixer:parse("2 * 3\n").
In prefixer:parse(String=2 * 3
)
{prefixer:13}: "Tokens" -> [{num,1,{2,[]}},{'*',1},{num,1,{3,[]}},{eoln,2}]
ParseTree='$undefined'
'$undefined'
2> prefixer:parse("2 + 3\n").
In prefixer:parse(String=2 + 3
)
{prefixer:13}: "Tokens" -> [{num,1,{2,[]}},{'+',1},{num,1,{3,[]}},{eoln,2}]
** exception error: {yecc_bug,"1.3",{missing_state_in_action_table,10}}
in function prefixer_parser:yeccpars2/7
in call from prefixer_parser:yeccpars0/2
in call from prefixer:parse/1


However, if I don't use precedence in the grammar, the parser will accept (but incorrectly parse) expressions containing '+' or '-' (e.g. 3 + 4 * 5 parses similarly to (3 + 4) * 5 instead of the correct 3 + (4 * 5)). No shift/reduce or reduce/reduce errors are reported by yecc in either case.

2). Also the Header field didn't seem to work for me, yecc didn't like it. If I remove the leading comment and uncomment the remaining lines I get the following error building my parser:

Quote:


# Generate the parser
erl -I -pa ./ebin -pa ./eunit/ebin -noshell -eval 'yecc:file("src/prefixer_parser"), halt().'
src/prefixer_parser.yrl:7: syntax error before: Nonterminals



The parser reads:

Code:

%% In spite of the documentation yecc does not recognize the Header field
%% Header "%% @Doc parses simple expressions."
%% "%% @Private"
%% "%% @Copyright 2009"
%% "%% @Version 0.1"

Nonterminals prog expr term.

Terminals '+' '-' '*' '/' '(' ')' eoln num var.

Rootsymbol prog.

%% Uncommenting the following lines prevents parsing  inputs using the
%%  + or - operators!
%Left 100 '+'.
%Left 100 '-'.
%Left 200 '*'.
%Left 200 '/'.


prog -> expr eoln.
prog -> prog expr eoln.

expr -> term.
expr -> expr '+' expr.
expr -> expr '-' expr.
expr -> expr '*' expr.
expr -> expr '/' expr.

term -> '(' expr ')'.
term -> num.
term -> var.



Have I made coding errors or are these yecc bugs?

Thanks:

Bill M
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rvirding
Posted: Thu Mar 19, 2009 10:37 pm Reply with quote
User Joined: 30 Aug 2006 Posts: 452 Location: Stockholm, Sweden
A quick question: in the code listing it looks like the first comment is broken into 2 lines; is this so or is just that the line is too long?
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