How do I get good error reporting for missing tokens in ANTLR 4?
I've written an ANTLR 4 grammar for a C-style programming language and am trying to improve error messages for missing delimiters, such as a missing comma or missing closing parenthesis in a function argument list.
I thought that ANTLR's single token insertion mechanism would accurately detect missing tokens, but instead I often get a "no viable alternative" error. Here is an example:
root
: expr+ EOF
;
expr
: '(' expr ')'
| '(' ')' '->' expr
| ID
;
ID: [a-zA-Z0-9$_]+;
Whitespace
: [ trnf]+ -> skip
;
Parsing input ( -> foo results in:
line 1:2 no viable alternative at input '(->'
Instead I'd like to see missing ')'. What's the recommended way to achieve this?
The only working solution I've found to date is to make all delimiters optional in the grammar and act on their absence in a visitor.
However, this feels like a workaround and has significant drawbacks that I'd rather avoid.
java parsing antlr antlr4
asked Nov 15 '18 at 8:22
Fred CurtsFred Curts
11
add a comment |
I've written an ANTLR 4 grammar for a C-style programming language and am trying to improve error messages for missing delimiters, such as a missing comma or missing closing parenthesis in a function argument list.
I thought that ANTLR's single token insertion mechanism would accurately detect missing tokens, but instead I often get a "no viable alternative" error. Here is an example:
root
: expr+ EOF
;
expr
: '(' expr ')'
| '(' ')' '->' expr
| ID
;
ID: [a-zA-Z0-9$_]+;
Whitespace
: [ trnf]+ -> skip
;
Parsing input ( -> foo results in:
line 1:2 no viable alternative at input '(->'
Instead I'd like to see missing ')'. What's the recommended way to achieve this?
The only working solution I've found to date is to make all delimiters optional in the grammar and act on their absence in a visitor.
However, this feels like a workaround and has significant drawbacks that I'd rather avoid.
java parsing antlr antlr4
asked Nov 15 '18 at 8:22
Fred CurtsFred Curts
11
add a comment |
I've written an ANTLR 4 grammar for a C-style programming language and am trying to improve error messages for missing delimiters, such as a missing comma or missing closing parenthesis in a function argument list.
I thought that ANTLR's single token insertion mechanism would accurately detect missing tokens, but instead I often get a "no viable alternative" error. Here is an example:
root
: expr+ EOF
;
expr
: '(' expr ')'
| '(' ')' '->' expr
| ID
;
ID: [a-zA-Z0-9$_]+;
Whitespace
: [ trnf]+ -> skip
;
Parsing input ( -> foo results in:
line 1:2 no viable alternative at input '(->'
Instead I'd like to see missing ')'. What's the recommended way to achieve this?
The only working solution I've found to date is to make all delimiters optional in the grammar and act on their absence in a visitor.
However, this feels like a workaround and has significant drawbacks that I'd rather avoid.
java parsing antlr antlr4
asked Nov 15 '18 at 8:22
Fred CurtsFred Curts
11
I've written an ANTLR 4 grammar for a C-style programming language and am trying to improve error messages for missing delimiters, such as a missing comma or missing closing parenthesis in a function argument list.
I thought that ANTLR's single token insertion mechanism would accurately detect missing tokens, but instead I often get a "no viable alternative" error. Here is an example:
root
: expr+ EOF
;
expr
: '(' expr ')'
| '(' ')' '->' expr
| ID
;
ID: [a-zA-Z0-9$_]+;
Whitespace
: [ trnf]+ -> skip
;
Parsing input ( -> foo results in:
line 1:2 no viable alternative at input '(->'
Instead I'd like to see missing ')'. What's the recommended way to achieve this?
The only working solution I've found to date is to make all delimiters optional in the grammar and act on their absence in a visitor.
However, this feels like a workaround and has significant drawbacks that I'd rather avoid.
java parsing antlr antlr4
java parsing antlr antlr4
asked Nov 15 '18 at 8:22
Fred CurtsFred Curts
11
asked Nov 15 '18 at 8:22
Fred CurtsFred Curts
11
edited Nov 15 '18 at 21:27
Fred Curts
asked Nov 15 '18 at 8:22
Fred CurtsFred Curts
11
asked Nov 15 '18 at 8:22
Fred CurtsFred Curts
11
asked Nov 15 '18 at 8:22
Fred CurtsFred Curts
11
11
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
The first thing to note is that you will get almost exactly the error message you want if you remove the '(' expr ')' rule:
expr
: '(' ')' '->' expr
| ID
;
Error message:
line 1:2 missing ')' at '->'
line 1:4 mismatched input '<EOF>' expecting {'(', ID}
I believe you get errors like "mismatched input X, expecting Y" and "missing X" for LL(1) grammars and no viable alternative when lookahead is required at the current position or something like that.
So with that in mind, we can try to rewrite your grammar to be LL(1):
expr
: '(' ( expr ')' | ')' '->' expr )
| ID
;
Then the error message becomes:
line 1:2 mismatched input '->' expecting {'(', ')', ID}
That's pretty close to what you want.
answered Nov 15 '18 at 21:25
sepp2ksepp2k
297k38597613
Thanks for your suggestion. Unfortunately, this refactoring doesn't get me close enough to what I want. In my real-world grammar, the ensuing error message contains many more than three expected tokens, and it isn't obvious at all from that message that a)is missing. (Another problem that I discovered is that this refactoring is incompatible with ANTLR 4's way of dealing with operator precedence, which seems to require top-level alternatives.)
– Fred Curts
Nov 15 '18 at 21:40
add a comment |
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1 Answer
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active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
The first thing to note is that you will get almost exactly the error message you want if you remove the '(' expr ')' rule:
expr
: '(' ')' '->' expr
| ID
;
Error message:
line 1:2 missing ')' at '->'
line 1:4 mismatched input '<EOF>' expecting {'(', ID}
I believe you get errors like "mismatched input X, expecting Y" and "missing X" for LL(1) grammars and no viable alternative when lookahead is required at the current position or something like that.
So with that in mind, we can try to rewrite your grammar to be LL(1):
expr
: '(' ( expr ')' | ')' '->' expr )
| ID
;
Then the error message becomes:
line 1:2 mismatched input '->' expecting {'(', ')', ID}
That's pretty close to what you want.
answered Nov 15 '18 at 21:25
sepp2ksepp2k
297k38597613
Thanks for your suggestion. Unfortunately, this refactoring doesn't get me close enough to what I want. In my real-world grammar, the ensuing error message contains many more than three expected tokens, and it isn't obvious at all from that message that a)is missing. (Another problem that I discovered is that this refactoring is incompatible with ANTLR 4's way of dealing with operator precedence, which seems to require top-level alternatives.)
– Fred Curts
Nov 15 '18 at 21:40
add a comment |
The first thing to note is that you will get almost exactly the error message you want if you remove the '(' expr ')' rule:
expr
: '(' ')' '->' expr
| ID
;
Error message:
line 1:2 missing ')' at '->'
line 1:4 mismatched input '<EOF>' expecting {'(', ID}
I believe you get errors like "mismatched input X, expecting Y" and "missing X" for LL(1) grammars and no viable alternative when lookahead is required at the current position or something like that.
So with that in mind, we can try to rewrite your grammar to be LL(1):
expr
: '(' ( expr ')' | ')' '->' expr )
| ID
;
Then the error message becomes:
line 1:2 mismatched input '->' expecting {'(', ')', ID}
That's pretty close to what you want.
answered Nov 15 '18 at 21:25
sepp2ksepp2k
297k38597613
Thanks for your suggestion. Unfortunately, this refactoring doesn't get me close enough to what I want. In my real-world grammar, the ensuing error message contains many more than three expected tokens, and it isn't obvious at all from that message that a)is missing. (Another problem that I discovered is that this refactoring is incompatible with ANTLR 4's way of dealing with operator precedence, which seems to require top-level alternatives.)
– Fred Curts
Nov 15 '18 at 21:40
add a comment |
The first thing to note is that you will get almost exactly the error message you want if you remove the '(' expr ')' rule:
expr
: '(' ')' '->' expr
| ID
;
Error message:
line 1:2 missing ')' at '->'
line 1:4 mismatched input '<EOF>' expecting {'(', ID}
I believe you get errors like "mismatched input X, expecting Y" and "missing X" for LL(1) grammars and no viable alternative when lookahead is required at the current position or something like that.
So with that in mind, we can try to rewrite your grammar to be LL(1):
expr
: '(' ( expr ')' | ')' '->' expr )
| ID
;
Then the error message becomes:
line 1:2 mismatched input '->' expecting {'(', ')', ID}
That's pretty close to what you want.
answered Nov 15 '18 at 21:25
sepp2ksepp2k
297k38597613
The first thing to note is that you will get almost exactly the error message you want if you remove the '(' expr ')' rule:
expr
: '(' ')' '->' expr
| ID
;
Error message:
line 1:2 missing ')' at '->'
line 1:4 mismatched input '<EOF>' expecting {'(', ID}
I believe you get errors like "mismatched input X, expecting Y" and "missing X" for LL(1) grammars and no viable alternative when lookahead is required at the current position or something like that.
So with that in mind, we can try to rewrite your grammar to be LL(1):
expr
: '(' ( expr ')' | ')' '->' expr )
| ID
;
Then the error message becomes:
line 1:2 mismatched input '->' expecting {'(', ')', ID}
That's pretty close to what you want.
answered Nov 15 '18 at 21:25
sepp2ksepp2k
297k38597613
answered Nov 15 '18 at 21:25
sepp2ksepp2k
297k38597613
answered Nov 15 '18 at 21:25
sepp2ksepp2k
297k38597613
answered Nov 15 '18 at 21:25
sepp2ksepp2k
297k38597613
297k38597613
Thanks for your suggestion. Unfortunately, this refactoring doesn't get me close enough to what I want. In my real-world grammar, the ensuing error message contains many more than three expected tokens, and it isn't obvious at all from that message that a)is missing. (Another problem that I discovered is that this refactoring is incompatible with ANTLR 4's way of dealing with operator precedence, which seems to require top-level alternatives.)
– Fred Curts
Nov 15 '18 at 21:40
add a comment |
Thanks for your suggestion. Unfortunately, this refactoring doesn't get me close enough to what I want. In my real-world grammar, the ensuing error message contains many more than three expected tokens, and it isn't obvious at all from that message that a)is missing. (Another problem that I discovered is that this refactoring is incompatible with ANTLR 4's way of dealing with operator precedence, which seems to require top-level alternatives.)
– Fred Curts
Nov 15 '18 at 21:40
Thanks for your suggestion. Unfortunately, this refactoring doesn't get me close enough to what I want. In my real-world grammar, the ensuing error message contains many more than three expected tokens, and it isn't obvious at all from that message that a
) is missing. (Another problem that I discovered is that this refactoring is incompatible with ANTLR 4's way of dealing with operator precedence, which seems to require top-level alternatives.)– Fred Curts
Nov 15 '18 at 21:40
Thanks for your suggestion. Unfortunately, this refactoring doesn't get me close enough to what I want. In my real-world grammar, the ensuing error message contains many more than three expected tokens, and it isn't obvious at all from that message that a
) is missing. (Another problem that I discovered is that this refactoring is incompatible with ANTLR 4's way of dealing with operator precedence, which seems to require top-level alternatives.)– Fred Curts
Nov 15 '18 at 21:40
add a comment |
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