Left recursion detected in .jj file

Question

Not sure what is causing this really. When I try to compile the file I get an error saying "Left recursion detected expression... -> fragment ... -> expression.

The area of code that has this is this section

void statement() : {}

{
    identifier() <ASSIGN> expression()
    | identifier() <ASSIGN> <STRING>
    | <EMARK> expression()
    | <QMARK> identifier()
    | identifier(arg_list())
    | <BEGIN>(statement() <SEMIC>)+ <END>
    | <IF> condition() <THEN> statement()
    | <IF> condition() <THEN> statement() <ELSE> statement()
    | <WHILE> (condition()) <DO> statement()
    | {}
}

void expression () : {}
{
    fragment()((<PLUS_SIGN> | <MINUS_SIGN> | <MULT_SIGN> | <DIV_SIGN>) fragment())*
}

void fragment () : {}
{
    identifier() | <NUM> | (<PLUS_SIGN> | <MINUS_SIGN>)fragment() | expression()
}

I'm not really sure how to go about fixing this problem and would appreciate any help with it! Thanks!

This assignment has been discussed in several recent stack overflow questions. Check out questions http://stackoverflow.com/questions/20287086/left-factoring-removing-left-recursion-javacc and http://stackoverflow.com/questions/20364288/left-recursion-elimination-in-an-ll1-grammar . — Theodore Norvell, Dec 10 '13 at 13:10

score 2 · Accepted Answer · answered Dec 11 '13 at 14:08

2

void fragment() #void : {}
|<LBR> expression() <RBR>

Change the fragment production rule, where it has expression to include brackets on either side. This should solve your recursion problem.

answered Dec 11 '13 at 14:08

Conor Murphy

90
1
8

score 1 · Answer 2 · answered Dec 09 '13 at 23:27

1

I think your abstraction is just a little off.

(<PLUS_SIGN> | <MINUS_SIGN>)fragment()

is more of an expression which can negate a fragment

I would consider

void expression () : {}
{
    fragment()((<PLUS_SIGN> | <MINUS_SIGN> | <MULT_SIGN> | <DIV_SIGN>) fragment())*
    | <MINUS_SIGN> fragment()
    | <PLUS_SIGN> fragment()
}

I also belive that fragment does not need to be an expression() either the recursion is already encapsulated by ((<PLUS_SIGN> | <MINUS_SIGN> | <MULT_SIGN> | <DIV_SIGN>) fragment())* being able to repeat

answered Dec 09 '13 at 23:27

Paperwaste

665
2
6
14

Thanks for the reply. I tried what you said and the error disapered, however I get 4 warnings now as follows: Warning: Choice conflict involving two expansions at line 243, column 11 and line 244, column 11 respectively. A common prefix is: Consider using a lookahead of 2 for earlier expansion. They are all similar to this, another one mentions a common prefix being if and not. Any ideas what this is telling me? – Ayohaych Dec 09 '13 at 23:43
1

No sorry not able to help you there i'm not familiar with .jj i was only able to help your recursion problem because i have a little experience with haskell. Goodluck – Paperwaste Dec 09 '13 at 23:57

Left recursion detected in .jj file

2 Answers2