I am trying to implement a grammar in Antlr4 for a simple template engine. This engine consists of 3 different clauses:
IF ANSWERED ( variable )
END IF
Variable
Variable can be any upper or lowercase letter including white spaces. Both IF ANSWERED and END IF are always uppercase.
I have written the following grammar/lexer rules so far, but my problem is that IF ANSWERED keeps getting recognized as a Variable and not as 2 tokens IF and ANSWERED.
grammar program;
/**grammar */
command: (ifStart | ifEnd | VARIABLE ) EOF;
ifStart: IF ANSWERED '(' VARIABLE ')';
ifEnd: 'END IF';
/** lexer */
IF: 'IF';
ANSWERED: 'ANSWERED';
TEXT: (LOWERCASE | UPPERCASE | NUMBER) ;
VARIABLE: (TEXT | [ \t\r\n])+;
fragment LOWERCASE: [a-z];
fragment UPPERCASE: [A-Z];
fragment NUMBER: [0-9];
If I try to parse IF ANSWERED ( FirstName ) I get the following output:
[#0,0:10='IF ANSWERED',**<VARIABLE>**,1:0]
[#1,11:11='(',<'('>,1:11]
[#2,12:25='Execution date',<VARIABLE>,1:12]
[#3,26:26=')',<')'>,1:26]
[#4,27:26='<EOF>',<EOF>,1:27]
line 1:0 mismatched input 'IF ANSWERED' expecting 'IF'
I read that Antlr4 is greedy and tries to match the biggest possible token, but I fail to understand what is the correct approach, or how to think through the problem to find a solution.
Correct: ANTLR's lexer is greedy, and tries to consume as much as possible. That is why IF ANSWERED is tokenised as a TEXT token instead of 2 separate keywords. You'll need to change TEXT so that it does not match spaces.
Something like this could get you started:
parse
: command* EOF
;
command
: (ifStatement | variable)+
;
ifStatement
: IF ANSWERED '(' variable ')' command* END IF
;
variable
: TEXT
;
IF : 'IF';
END : 'END';
ANSWERED : 'ANSWERED';
TEXT : [a-zA-Z0-9]+;
SPACES : [ \t\r\n]+ -> skip;
Related
I would like to solve the following ambiguity:
grammar test;
WS : (' ' | '\t' | '\n' | '\r' | '\f')+ -> skip;
program
:
input* EOF;
input
: '%' statement
| inputText
;
inputText
: ~('%')+
;
statement
: Identifier '=' DecimalConstant ';'
;
DecimalConstant
: [0-9]+
;
Identifier
: Letter LetterOrDigit*
;
fragment
Letter
: [a-zA-Z$##_.]
;
fragment
LetterOrDigit
: [a-zA-Z0-9$##_.]
;
Sample input:
%a=5;
aa bbbb
As soon as I put a space after "aa" with values like "bbbb" an ambiguity is created.
In fact I want inputText to contain the full string "aa bbbb".
There is no ambiguity. The input aa bbbb will always be tokenised as 2 Identifier tokens. No matter what any parser rule is trying to match. The lexer operates independently from the parser.
Also, the rule:
inputText
: ~('%')+
;
does not match one or more characters other than '%'.
Inside parser rules, the ~ negates tokens, not characters. So ~'%' inside a parser rule will match any token, other than a '%' token. Inside the lexer, ~'%' matches any character other than '%'.
But creating a lexer rule like this:
InputText
: ~('%')+
;
will cause your example input to be tokenised as a single '%' token, followed by a large 2nd token that'd match this: a=5;\naa bbbb. This is how ANTLR's lexer works: match as much characters as possible (no matter what the parser rule is trying to match).
I found the solution:
grammar test;
WS : (' ' | '\t' | '\n' | '\r' | '\f')+ -> skip;
program
:
input EOF;
input
: inputText ('%' statement inputText)*
;
inputText
: ~('%')*
;
statement
: Identifier '=' DecimalConstant ';'
;
DecimalConstant
: [0-9]+
;
Identifier
: Letter LetterOrDigit*
;
fragment
Letter
: [a-zA-Z$##_.]
;
fragment
LetterOrDigit
: [a-zA-Z0-9$##_.]
;
I want to write a grammar using Antlr4 that will parse a some definition but I've been struggling to get Antlr to co-operate.
The definition has two kinds of lines, a type and a property. I can get my grammar to parse the type line correctly but it either ignores the property lines or fails to identify PROPERTY_TYPE depending on how I tweak my grammar.
Here is my grammar (attempt # 583):
grammar TypeDefGrammar;
start
: statement+ ;
statement
: type NEWLINE
| property NEWLINE
| NEWLINE ;
type
: TYPE_KEYWORD TYPE_NAME; // e.g. 'type MyType1'
property
: PROPERTY_NAME ':' PROPERTY_TYPE ; // e.g. 'someProperty1: int'
TYPE_KEYWORD
: 'type' ;
TYPE_NAME
: IDENTIFIER ;
PROPERTY_NAME
: IDENTIFIER ;
PROPERTY_TYPE
: IDENTIFIER ;
fragment IDENTIFIER
: (LETTER | '_') (LETTER | DIGIT | '_' )* ;
fragment LETTER
: [a-zA-Z] ;
fragment DIGIT
: [0-9] ;
NEWLINE
: '\r'? '\n' ;
WS
: [ \t] -> skip ;
Here is a sample input:
type SimpleType
intProp1: int
stringProp2 : String
(returns the type but ignores intProp1, stringProp2.)
What am I doing wrong?
Usually when a rule does not match the whole input, but does match a prefix of it, it will simply match that prefix and leave the rest of the input in the stream without producing an error. If you want your rule to always match the whole input, you can add EOF to the end of the rule. That way you'll get proper error messages when it can't match the entire input.
So let's change your start rule to start : statement+ EOF;. Now applying start to your input will lead to the following error messages:
line 3:0 extraneous input 'intProp1' expecting {, 'type', PROPERTY_NAME, NEWLINE}
line 4:0 extraneous input 'stringProp2' expecting {, 'type', PROPERTY_NAME, NEWLINE}
So apparently intProp1 and stringProp2 aren't recognized as PROPERTY_NAMEs. So let's look at which tokens are generated (you can do that using the -tokens option to grun or by just iterating over the token stream in your code):
[#0,0:3='type',<'type'>,1:0]
[#1,5:14='SimpleType',<TYPE_NAME>,1:5]
[#2,15:15='\n',<NEWLINE>,1:15]
[#3,16:16='\n',<NEWLINE>,2:0]
[#4,17:24='intProp1',<TYPE_NAME>,3:0]
[#5,25:25=':',<':'>,3:8]
[#6,27:29='int',<TYPE_NAME>,3:10]
[#7,30:30='\n',<NEWLINE>,3:13]
[#8,31:41='stringProp2',<TYPE_NAME>,4:0]
[#9,43:43=':',<':'>,4:12]
[#10,45:50='String',<TYPE_NAME>,4:14]
[#11,51:51='\n',<NEWLINE>,4:20]
[#12,52:51='<EOF>',<EOF>,5:0]
So all of the identifiers in the code are recognized as TYPE_NAMEs, not PROPERTY_NAMEs. In fact, it is not clear what should distinguish a TYPE_NAME from a PROPERTY_NAME, so now let's actually look at your grammar:
TYPE_NAME
: IDENTIFIER ;
PROPERTY_NAME
: IDENTIFIER ;
PROPERTY_TYPE
: IDENTIFIER ;
fragment IDENTIFIER
: (LETTER | '_') (LETTER | DIGIT | '_' )* ;
Here you have three lexer rules with exactly the same definition. That's a bad sign.
Whenever multiple lexer rules can match on the current input, ANTLR chooses the one that would produce the longest match, picking the one that comes first in the grammar in case of ties. This is known as the maximum munch rule.
If you have multiple rules with the same definition, that means those rules will always match on the same input and they will always produce matches of the same length. So by the maximum much rule, the first definition (TYPE_NAME) will always be used and the other ones might as well not exist.
The problem basically boils down to the fact that there's nothing that lexically distinguishes the different types of names, so there's no basis on which the lexer could decide which type of name a given identifier represents. That tells us that the names should not be lexer rules. Instead IDENTIFIER should be a lexer rule and the FOO_NAMEs should either be (somewhat unnecessary) parser rules or removed altogether (you can just use IDENTIFIER wherever you're currently using FOO_NAME).
grammar TestGrammar;
AND : 'AND' ;
OR : 'OR'|',' ;
NOT : 'NOT' ;
LPAREN : '(' ;
RPAREN : ')' ;
DQUOTE : '"' ;
WORD : [a-z0-9._#+=]+(' '[a-z0-9._#+=]+)* ;
WS : [ \t\r\n]+ -> skip ;
quotedword : DQUOTE WORD DQUOTE;
expression
: LPAREN expression+ RPAREN
| expression (AND expression)+
| expression (OR expression)+
| expression (NOT expression)+
| NOT expression+
| quotedword
| WORD;
I've managed to implement the above grammar for antlr4.
I've got a long way to go but for now my question is,
how can I make WORD generic? Basically I want this [a-z0-9._#+=] to be anything except the operators (AND, OR, NOT, LPAREN, RPAREN, DQUOTE, SPACE).
The lexer will use the first rule that can match the given input. Only if that rule can't match it, it will try the next one.
Therefore you can make your WORD rule generic by using this grammar:
AND : 'AND' ;
OR : 'OR'|',' ;
NOT : 'NOT' ;
LPAREN : '(' ;
RPAREN : ')' ;
DQUOTE : '"' ;
WS : [ \t\r\n]+ -> skip ;
WORD: .+? ;
Make sure to use the non-greedy operator ? in this case becaue otherwise once invoked the WORD rule will consume all following input.
As WORD is specified last, input will only be tried to be consumed by it if all previous lexer rules (all that have been defined above in the source code) have failed.
EDIT: If you don't want your WORD rule to match any input then you just have to modify the rule I provided. But the essence of my answer is that in the lexer you don't have to worry about two rules potentially matching the same input as long as you got the order in the source code right.
Try something like this grammar:
grammar TestGrammar;
...
WORD : Letter+;
QUOTEDWORD : '"' (~["\\\r\n])* '"' // disallow quotes, backslashes and crlf in literals
WS : [ \t\r\n]+ -> skip ;
fragment Letter :
[a-zA-Z$_] // these are the "java letters" below 0x7F
| ~[\u0000-\u007F\uD800-\uDBFF] // covers all characters above 0x7F which are not a surrogate
| [\uD800-\uDBFF] [\uDC00-\uDFFF] // covers UTF-16 surrogate pairs encodings for U+10000 to U+10FFFF
;
expression:
...
| QUOTEDWORD
| WORD+;
Maybe you want to use escape sequences in QUOTEDWORD, then look in this example how to do this.
This grammar allows you:
to have quoted words interpreted as string literals (preserving all spaces within)
to have multiple words separated by whitespace (which is ignored)
in a little test-parser I just wrote, I encountered a weird problem, which I don't quite understand.
Stripping it down to the smallest example showing the problem, let's start with the following grammar:
Testing.g4:
grammar Testing;
cscript // This is the construct I shortened
: (statement_list)* ;
statement_list
: statement ';' statement_list?
| block
;
statement
: assignment_statement
;
block : '{' statement_list? '}' ;
expression
: left=expression op=('*'|'/') right=expression # arithmeticExpression
| left=expression op=('+'|'-') right=expression # arithmeticExpression
| left=expression op=Comparison_operator right=expression # comparisonExpression
| ID # variableValueExpression
| constant # ignore // will be executed with the rule name
;
assignment_statement
: ID op=Assignment_operator expression
;
constant
: INT
| REAL;
Assignment_operator : ('=' | '+=' | '-=') ;
Comparison_operator : ('<' | '>' | '==' | '!=') ;
Comment : '//' .*? '\n' -> skip;
fragment NUM : [0-9];
INT : NUM+;
REAL
: NUM* '.' NUM+
| '.' NUM+
| INT
;
ID : [a-zA-Z_] [a-zA-Z_0-9]*;
WS : [ \t\r\n]+ -> skip;
Using the input
z = x + y;
everything is fine, we get a parse tree which goes from cscript to statement_list, statement, assignment_statement, id and expression. Great!
Now, if I add the possibility to declare variables, all goes down the drain:
This is the change to the grammar:
cscript
: (statement_list | variable_declaration ';')* ;
variable_declaration
: type ID ('=' expression)?
;
type
: 'int'
| 'real'
;
statement_list
: statement ';' statement_list?
| block
;
statement
: assignment_statement
;
// (continue as before)
All of a sudden, the same test-input gets wrongly dissected into two statement_lists, each continued to a statement with a "missing ';'" warning, the first going to an incomplete assignment_statement of "z =" and the second to an incomplete assignment_statement "x +".
My attempt to show the parse tree in text-form:
cscript
statement_list
statement
assignment_statement
'z'
'=' [marked as error]
[warning: missing ';']
statement_list
statement
assignment_statement
'x'
'+' [marked as error]
'y' [marked as error]
';'
Can anyone tell me what the problem is? (And how to fix it? ;-))
Edit on 2016-12-26, after Mike's comment:
After replacing all implicit lexer rules with explicit declarations, all of a sudden, the input "z = x + y" worked. (thumbs up)
The next thing I did was restoring more of the original example I had in mind, and adding a new input line
int x = 22;
to the input (which worked previously, but did not make it into the minimal example). Now, that line fails. This is the -token output of the test rig:
[#0,0:2='int',<4>,1:0]
[#1,4:4='x',<22>,1:4]
[#2,6:6='=',<1>,1:6]
[#3,8:9='22',<20>,1:8]
[#4,10:10=';',<12>,1:10]
[#5,13:13='z',<22>,2:0]
[#6,15:15='=',<1>,2:2]
[#7,17:17='x',<22>,2:4]
[#8,19:19='+',<18>,2:6]
[#9,21:21='y',<22>,2:8]
[#10,22:22=';',<12>,2:9]
[#11,25:24='<EOF>',<-1>,3:0]
line 1:6 mismatched input '=' expecting '='
As the problem seemed to be in the variable_declaration part, I even tried to split this into two parsing rules like this:
cscript
: (statement_list | variable_declaration_and_assignment SEMICOLON | variable_declaration SEMICOLON)* ;
variable_declaration_and_assignment
: type ID EQUAL expression
;
variable_declaration
: type ID
;
With the result:
line 1:6 no viable alternative at input 'intx='
Still stuck :-(
BTW: Splitting the "int x = 22;" into "int x;" and "x = 22;" works. sigh
Edit on 2016-12-26, after Mike's next comment:
Double-checked, and everything is lexer rules. Still, the mismatch between '=' and '=' (which I unfortunately cannot reconstruct anymore) gave me the idea to check the token types. The current status is:
(Shortened grammar)
cscript
: (statement_list | variable_declaration)* ;
...
variable_declaration
: type ID (EQUAL expression)? SEMICOLON
;
...
Assignment_operator : (EQUAL | PLUS_EQ | MINUS_EQ) ;
// among others
PLUS_EQ : '+=';
MINUS_EQ : '-=';
EQUAL: '=';
...
Shortened output:
[#0,0:2='int',<4>,1:0]
[#1,4:4='x',<22>,1:4]
[#2,6:6='=',<1>,1:6]
...
line 1:6 mismatched input '=' expecting ';'
Here, if I understand this correctly, the '=' is parsed to token type 1, which - according to the lexer.tokens output - is Assignment_Operator, while the expected EQUAL would be 13.
Might this be the problem?
Ok, seems the main take away here is: think about your definitions and how you define them. Create explicit lexer rules for your literals instead of defining them implicitly in the parser rules. Check the token values you get from the lexer if the parser gives you weird errors, because they must be correct in the first place or your parse has no chance to do its job.
I have the following grammar:
grammar Aligner;
line
: emptyLine
| codeLine
;
emptyLine
: ( KW_EMPTY KW_LINE )?
( EOL | EOF )
;
codeLine
: KW_LINE COLON
indent
CODE
( EOL | EOF )
;
indent
: absolute_indent
| relative_indent
;
absolute_indent
: NUMBER
;
relative_indent
: sign NUMBER
;
sign
: PLUS
| MINUS
;
COLON: ':';
MINUS: '-';
PLUS: '+';
KW_EMPTY: 'empty';
KW_LINE: 'line';
NUMBER
: DIGIT+
;
EOL
: ('\n' | '\r\n')
;
SPACING
: LINE_WS -> skip
;
CODE
: (~('\n' | '\r'))+
;
fragment
DIGIT
: '0'..'9'
;
fragment
LINE_WS
: ' '
| '\t'
| '\u000C'
;
when I try to parse - empty line I receive error: line 1:0 no viable alternative at input 'empty line'. When I debug what is going on, the very first token is from type CODE and includes the whole line.
What I am doing wrong?
ANTLR will try to match the longest possible token. When two lexer rules match the same string of a given length, the first rule that appears in the grammar wins.
You rule CODE is basically a catch-all: it will match whole lines of text. So here ANTLR has the choice of matching empty line as one single token of type CODE, and as no other rule can produce a token of length 10, the CODE rule will consume the whole line.
You should rewrite the CODE rule to make it match only what you mean by a code. Right now it's way too broad.