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Question concerning "IF", "ELSE IF", "ELSE" structures [message #1765892]

Wed, 14 June 2017 07:51

Eclipse User

Hi there,

currently I'm working on a grammar for a simple scripting language. Now I'm stuck at a structure like

IF (A == B) THEN 
  (...)
ELSE IF (A == C) THEN 
  (...)
ELSE
  IF (C == E) THEN 
    (...)
  ELSE IF (C == F) THEN 
    (...)
  ELSE
    (...)
  ENDIF  
  (...)
ENDIF

I end up with the message

error(211): ../org.xtext.example.iftest/src-gen/org/xtext/example/iftest/parser/antlr/internal/InternalIfTest.g:1009:3: [fatal] rule ruleCondition has non-LL(*) decision due to recursive rule invocations reachable from alts 1,2.  Resolve by left-factoring or using syntactic predicates or using backtrack=true option.

but in fact I don't have an idea how to rewrite this in order to work. I guess the main problem is the fact there's no way to distinguish "ELSE IF" from "ELSE" and "IF".

My grammar is appended.

Any help is appreciated.

Thanks in advance,

Christoph

Attachment: IfTest.xtext
(Size: 0.98KB, Downloaded 147 times)

Re: Question concerning "IF", "ELSE IF", "ELSE" structures [message #1765897 is a reply to message #1765892]

Wed, 14 June 2017 08:13

Eclipse User

did you read: https://zarnekow.blogspot.de/2012/10/xtext-corner-5-backtracking-vs.html and https://dslmeinte.wordpress.com/2011/12/05/using-syntactic-predicates-in-xtext-part-1/

Re: Question concerning "IF", "ELSE IF", "ELSE" structures [message #1765899 is a reply to message #1765897]

Wed, 14 June 2017 08:35

Eclipse User

Hi,

if understand those blog entries I can "guide" the parser by using the syntactic predicate. In fact the message does not appear if I write

Condition hidden(WS):
	{Condition}
	'IF' '('if=Expression')' 'THEN'
		(cond+=Condition)*
	(('ELSE' 'IF') '('elseif=Expression')' 'THEN'
		(cond+=Condition)*)*	
	(=> 'ELSE'
		(cond+=Condition)*)?	
	'ENDIF'	
;

but this results in an error when testing the intended structure like

IF (A == B) THEN 
  (...)
ELSE IF (A == C) THEN 
  (...)
ELSE
  IF (C == E) THEN 
    (...)
  ELSE IF (C == F) THEN 
    (...)
  ELSE
    (...)
  ENDIF  
  (...)
ENDIF

BTW the problem does not appear if the "ELSE IF" could be written "ELSEIF" (not possible, unfortunately). Do I have to structure the "ELSE IF"/"ELSE" part in a different way?

Thanks in advance for your patience,

Christoph

Re: Question concerning "IF", "ELSE IF", "ELSE" structures [message #1765900 is a reply to message #1765899]

Wed, 14 June 2017 08:39

Eclipse User

no more like a

Condition hidden(WS):
{Condition}
'IF' '('if=Expression')' 'THEN'
(cond+=Condition)*
(=> 'ELSE'
(cond+=Condition)*)?
'ENDIF'
;

Re: Question concerning "IF", "ELSE IF", "ELSE" structures [message #1765902 is a reply to message #1765900]

Wed, 14 June 2017 08:46

Eclipse User

p.s.

what are your requirements to parse else if

is it a if inside a else
or is it a elseif

Re: Question concerning "IF", "ELSE IF", "ELSE" structures [message #1765905 is a reply to message #1765900]

Wed, 14 June 2017 08:52

Eclipse User

Hi

This is an infamous problem for which the C language had an excuse; it predated YACC-like grammar tools. https://en.wikipedia.org/wiki/Dangling_else

Nowadays LALR tools should always help you redesign your grammar. Xtext is not LALR but I understand that ANTLR has some related facilities behind the scenes. IMHO syntactic predicates are a sticking plaster to avoid competent design.

You have an ENDIF that should avoid the usual problem but ELSE IF reintroduces it. I suspect that ELSE THEN, or ELSEIF should cure it.

Regards

Ed Willink

Re: Question concerning "IF", "ELSE IF", "ELSE" structures [message #1765910 is a reply to message #1765902]

Wed, 14 June 2017 08:53

Eclipse User

Hi,

the "ELSE IF" is an "ELSEIF", so there's a difference between

IF (....) THEN
ELSE
  IF (....) THEN
  ENDIF
ENDIF

and

IF (....) THEN
ELSE IF (....) THEN
ENDIF

Christoph

Re: Question concerning "IF", "ELSE IF", "ELSE" structures [message #1765911 is a reply to message #1765910]

Wed, 14 June 2017 08:54

Eclipse User

is this an indentation based language?

Re: Question concerning "IF", "ELSE IF", "ELSE" structures [message #1765915 is a reply to message #1765911]

Wed, 14 June 2017 09:14

Eclipse User

if yes maybe then you should use the whitespace feature of xtext instead of mimic it

Model:
	cs+=IfStatement*;
	
	
Literal:
	value=INT
;

IfStatement:
	{Condition}
	'IF' '('if+=Literal')' 'THEN' BEGIN
		expr+=Expression+
	END
	('ELSE' 'IF'  '('if+=Literal')' 'THEN' BEGIN expr+=Expression+ END)*
	( 'ELSE' BEGIN expr+=Expression+ END)?
	'ENDIF'	
;

Expression:
    IfStatement | Literal;

terminal BEGIN: 'synthetic:BEGIN';
terminal END: 'synthetic:END';

Re: Question concerning "IF", "ELSE IF", "ELSE" structures [message #1765918 is a reply to message #1765915]

Wed, 14 June 2017 09:22

Eclipse User

Hi

It may be your intention that an ELSE IF is an ELSEIF but that is the ambiguity. It is also an ELSE (IF ...). I strongly recommend that you use yacc / bison / LPG to check your grammar; intuitive intentions do not work. For a small example manual conversion will do. For larger examples, there is an Xtext2LPG transformation to assist in OCL grammar checking as part of the OCL project releng tooling.

Regards

Ed Willink

Re: Question concerning "IF", "ELSE IF", "ELSE" structures [message #1765932 is a reply to message #1765915]

Wed, 14 June 2017 09:52

Eclipse User

Hi,

unfortunately it is not identation aware.

Christoph

Re: Question concerning "IF", "ELSE IF", "ELSE" structures [message #1765935 is a reply to message #1765932]

Wed, 14 June 2017 09:55

Eclipse User

well for if else it is?!?

Re: Question concerning "IF", "ELSE IF", "ELSE" structures [message #1765936 is a reply to message #1765918]

Wed, 14 June 2017 09:55

Eclipse User

Hi,

could this

Quote:

It may be your intention that an ELSE IF is an ELSEIF but that is the ambiguity. It is also an ELSE (IF ...).

lead to the fact that there will be no solution in Xtext apart from changing "ELSE IF" to "ELSEIF"?

Christoph

Re: Question concerning "IF", "ELSE IF", "ELSE" structures [message #1765937 is a reply to message #1765936]

Wed, 14 June 2017 09:58

Eclipse User

well i dont know what happens if you partially apply the indentation thing and hide the synthethic tokens elsewhere

Re: Question concerning "IF", "ELSE IF", "ELSE" structures [message #1765940 is a reply to message #1765937]

Wed, 14 June 2017 10:15

Eclipse User

Hi

This is not an Xtext problem. When you design a bad grammar it means that your users are going to encounter surprises. (C programmers have to experience pain to learn about the bad else problem. Or they mindlessly follow a corporate style guide that mandates gratuitous {}.) You need to rethink the grammar. I suggested ELSEIF or ELSE THEN.

Regards

Ed Willink

Re: Question concerning "IF", "ELSE IF", "ELSE" structures [message #1765967 is a reply to message #1765940]

Wed, 14 June 2017 11:53

Eclipse User

Hi,

thanks for your advice. The indention approach seems to solve the problem, in addition this would greatly increase the readability. The change of "ELSE IF" to "ELSEIF" has to be discussed with the development team. As stated already this would also lead to the desired structure.

Thanks to all of you,

Christoph

p.s.: Is there am way to mark this as solved?

Re: Question concerning "IF", "ELSE IF", "ELSE" structures [message #1765980 is a reply to message #1765892]

Wed, 14 June 2017 12:45

Eclipse User

>p.s.: Is there am way to mark this as solved?
nope, not in this forum

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