I begin with an otherwise well formed (and well working) grammar for a language. Variables,
binary operators, function calls, lists, loops, conditionals, etc. To this grammar I'd like to add what I'm calling the object construct:
object
: object_name ARROW more_objects
;
more_objects
: object_name
| object_name ARROW more_objects
;
object_name
: IDENTIFIER
;
The point is to be able to access scalars nested in objects. For example:
car->color
monster->weapon->damage
pc->tower->motherboard->socket_type
I'm adding object as a primary_expression:
primary_expression
: id_lookup
| constant_value
| '(' expression ')'
| list_initialization
| function_call
| object
;
Now here's a sample script:
const list = [ 1, 2, 3, 4 ];
for var x in list {
send "foo " + x + "!";
}
send "Done!";
Prior to adding the nonterminal object as a primary_expression everything is sunshine and puppies. Even after I add it, Bison doesn't complain. No shift and/or reduce conflicts reported. And the generated code compiles without a sound. But when I try to run the sample script above, I get told error on line 2: Attempting to use undefined symbol '{' on line 2.
If I change the script to:
var list = 0;
for var x in [ 1, 2, 3, 4 ] {
send "foo " + x + "!";
}
send "Done!";
Then I get error on line 3: Attempting to use undefined symbol '+' on line 3.
Clearly the presence of object in the grammar is messing up how the parser behaves [SOMEhow], and I feel like I'm ignoring a rather simple principle of language theory that would fix this in a jiff, but the fact that there aren't any shift/reduce conflicts has left me bewildered.
Is there a better way (grammatically) to write these rules? What am I missing? Why aren't there any conflicts?
(And here's the full grammar file in case it helps)
UPDATE: To clarify, this language, which compiles into code being run by a virtual machine, is embedded into another system - a game, specifically. It has scalars and lists, and there are no complex data types. When I say I want to add objects to the language, that's actually a misnomer. I am not adding support for user-defined types to my language.
The objects being accessed with the object construct are actually objects from the game which I'm allowing the language processor to access through an intermediate layer which connects the VM to the game engine. This layer is designed to decouple as much as possible the language definition and the virtual machine mechanics from the implementation and details of the game engine.
So when, in my language I write:
player->name
That only gets codified by the compiler. "player" and "name" are not traditional identifiers because they are not added to the symbol table, and nothing is done with them at compile time except to translate the request for the name of the player into 3-address code.
It seems you are doing a classical error when using direct strings in the yacc source file. Since you are using a lexer, you can only use token names in yacc source files. More on this here
So I spent a reasonable amount of time picking over the grammar (and the bison output) and can't see what is obviously wrong here. Without having the means to execute it, I can't easily figure out what is going on by experimentation. Therefore, here are some concrete steps I usually go through when debugging grammars. Hopefully you can do any of these you haven't already done and then perhaps post follow-ups (or edit your question) with any results that might be revealing:
Contrive the smallest (in terms of number of tokens) possible working input, and the smallest possible non-working inputs based on the rules you expect to be applied.
Create a copy of the grammar file including only the troublesome rules and as few other supporting rules as you can get away with (i.e. you want a language that only allows construction of sequences consisting of the object and more_object rules, joined by ARROW. Does this work as you expect?
Does the rule in which it is nested work as you expect? Try replacing object with some other very simple rule (using some tokens not occuring elsewhere) and seeing if you can include those tokens without it breaking everything else.
Run bison with --report=all. Inspect the output to try to trace the rules you've added and the states that they affect. Try removing those rules and repeat the process - what has changed? This is extremely time consuming often, and is a giant pain, but it's a good last resort. I recommend a pencil and some paper.
Looking at the structure of your error output - '+' is being recognised as an identifier token, and is therefore being looked up as a symbol. It might be worth checker your lexer to see how it is processing identifier tokens. You might just accidentally be grabbing too much. As a further debugging technique, you might consider turning some of those token literals (e.g. '+', '{', etc) into real tokens so that bison's error reporting can help you out a little more.
EDIT: OK, the more I've dug into it, the more I'm convinced that the lexer is not necessarily working as it should be. I would double-check that the stream of tokens you are getting from yylex() matches your expectations before proceeding any further. In particular, it looks like a bunch of symbols that you consider special (e.g. '+' and '{') are being captured by some of your regular expressions, or at least are being allowed to pass for identifiers.
You don't get shift/reduce conflicts because your rules using object_name and more_objects are right-recursive - rather than the left-recursive rules that Yacc (Bison) handles most naturally.
On classic Yacc, you would find that you can run out of stack space with deep enough nesting of the 'object->name->what->not' notation. Bison extends its stack at runtime, so you have to run out of memory, which is a lot harder these days than it was when machines had a few megabytes of memory (or less).
One result of the right-recursion is that no reductions occur until you read the last of the object names in the chain (or, more accurately, one symbol beyond that). I see that you've used right-recursion with your statement_list rule too - and in a number of other places too.
I think your principal problem is that you failed to define a subtree constructor
in your object subgrammar. (EDIT: OP says he left the semantic actions for
object out of his example text. That doesn't change the following answer).
You probably have to lookup up the objects in the order encountered, too.
Maybe you intended:
primary_expression
: constant_value { $$ = $1; }
| '(' expression ')' { $$ = $2; }
| list_initialization { $$ = $1; }
| function_call { $$ = $1; }
| object { $$ = $1; }
;
object
: IDENTIFIER { $$ = LookupVariableOrObject( yytext ); }
| object ARROW IDENTIFIER { $$ = LookupSubobject( $1, yytext ); }
;
I assume that if one encounters an identifier X by itself, your default interpretation
is that it is a variable name. But, if you encounter X -> Y, then even if X
is a variable name, you want the object X with subobject Y.
What LookupVarOrObject does is to lookup the leftmost identifier encountered to see if it is variable
(and return essentially the same value as idlookup which must produce an AST node of type AST_VAR),
or see if it is valid object name, and return an AST node marked as an AST_OBJ,
or complain if the identifier isn't one of these.
What LookupSuboject does, is to check its left operand to ensure it is an AST_OBJ
(or an AST_VAR whose name happens to be the same as that of an object).
and complain if it is not. If it is, then its looks up the yytext-child object of
the named AST_OBJ.
EDIT: Based on discussion comments in another answer, right-recursion in the OP's original
grammar might be problematic if the OP's semantic checks inspect global lexer state (yytext).
This solution is left-recursive and won't run afoul of that particular trap.
id_lookup
: IDENTIFIER
is formally identical to
object_name
: IDENTIFIER
and object_name would accept everything that id_lookup wouldn't, so assertLookup( yytext ); probably runs on everything that may look like IDENTIFIER and is not accepted by enother rule just to decide between the 2 and then object_name can't accept because single lookahead forbids that.
For the twilight zone, the two chars that you got errors for are not declared as tokens with opends the zone of undefinded behavior and could trip parser into trying to treat them as potential identifiers when the grammar gets loose.
I just tried running muscl in Ubuntu 10.04 using bison 2.4.1 and I was able to run both of your examples with no syntax errors. My guess is that you have a bug in your version of bison. Let me know if I'm somehow running your parser wrong. Below is the output from the first example you gave.
./muscle < ./test1.m (this was your first test)
\-statement list
|-declaration (constant)
| |-symbol reference
| | \-list (constant)
| \-list
| |-value
| | \-1
| |-value
| | \-2
| |-value
| | \-3
| \-value
| \-4
|-loop (for-in)
| |-symbol reference
| | \-x (variable)
| |-symbol reference
| | \-list (constant)
| \-statement list
| \-send statement
| \-binary op (addition)
| |-binary op (addition)
| | |-value
| | | \-foo
| | \-symbol reference
| | \-x (variable)
| \-value
| \-!
\-send statement
\-value
\-Done!
+-----+----------+-----------------------+-----------------------+
| 1 | VALUE | 1 | |
| 2 | ELMT | #1 | |
| 3 | VALUE | 2 | |
| 4 | ELMT | #3 | |
| 5 | VALUE | 3 | |
| 6 | ELMT | #5 | |
| 7 | VALUE | 4 | |
| 8 | ELMT | #7 | |
| 9 | LIST | | |
| 10 | CONST | #10 | #9 |
| 11 | ITER_NEW | #11 | #10 |
| 12 | BRA | #14 | |
| 13 | ITER_INC | #11 | |
| 14 | ITER_END | #11 | |
| 15 | BRT | #22 | |
| 16 | VALUE | foo | |
| 17 | ADD | #16 | #11 |
| 18 | VALUE | ! | |
| 19 | ADD | #17 | #18 |
| 20 | SEND | #19 | |
| 21 | BRA | #13 | |
| 22 | VALUE | Done! | |
| 23 | SEND | #22 | |
| 24 | HALT | | |
+-----+----------+-----------------------+-----------------------+
foo 1!
foo 2!
foo 3!
foo 4!
Done!
Related
I'm a freshman in Bison and Compiler principles, now I'm trying to write an easy Verilog parser in Flex&Bison according to the IEEE Standard. Here is the question: when a grammar's body has optional parts, which are enclosed by square brackets, what should I do in Bison?
The BNF grammar maybe like this:
input_declaration ::= input [ net_type ] [ signed ] [ range ] list_of_port_identifiers
1, Should I enumerate them one by one like the following?
input_declaration : INPUT list_of_port_identifiers
| INPUT net_type list_of_port_identifiers
| INPUT signed list_of_port_identifiers
| INPUT net_type signed list_of_port_identifiers
....
This way can do solve the problem, but I feel it's so stupid.
2, Should I write a %empty directive in the optional parts' grammar like the following?
net_type:
%empty
| SUPPLY0
| SUPPLY1
| TRI
| TRIAND
| TRIOR
| WIRE
| WAND
| WOR
;
But I think this will cause some conflicts. So what is the best idea for this?
The best solution is (2):
net_type: %empty | ...;
signed: %empty | ...;
range: %empty | ...;
You should not see any conflicts between them unless they share tokens.
If you attempted to write all combinations as (1) and it worked, I still believe that you probably left out one combination by mistake, as Bison would in any case detect the conflict as in (2) and warn you.
I'm having a minor problem in trying to figure out how to resolve a conflict in my CUP parser project. I understand why the error is occurring, VariableDeclStar's first terminal can be ID, as well as Type, which brings up the conflict, however I cannot figure out how to resolve the conflict in a way that would preserve Type and Variable as separate states. Any help or tips would be appreciated.
VariableDecl ::= Variable SEMICOLON {::};
Variable ::= Type ID {::};
Type ::= INT {::}
| DOUBLE {::}
| BOOLEAN {::}
| STRING {::}
| Type LEFTBRACKET RIGHTBRACKET {::}
| ID {::};
VariableDeclStar::= VariableDecl VariableDeclStar {::}
| {::};
https://i.gyazo.com/0ac3fbf4ebc2d3968f1c2a78c292bc0d.png
In this term, I have course on Compilers and we are currently studying syntax - different grammars and types of parsers. I came across a problem which I can't exactly figure out, or at least I can't make sure I'm doing it correctly. I already did 2 attempts and counterexamples were found.
I am given this ambiguous grammar for arithmetic expressions:
E → E+E | E-E | E*E | E/E | E^E | -E | (E)| id | num , where ^ stands for power.
I figured out what the priorities should be. Highest priority are parenthesis, followed by power, followed by unary minus, followed by multiplication and division, and then there is addition and substraction. I am asked to convert this into equivalent LL(1) grammar. So I wrote this:
E → E+A | E-A | A
A → A*B | A/B | B
B → -C | C
C → D^C | D
D → (E) | id | num
What seems to be the problem with this is not equivalent grammar to the first one, although it's non-ambiguous. For example: Given grammar can recognize input: --5 while my grammar can't. How can I make sure I'm covering all cases? How should I modify my grammar to be equivalent with the given one? Thanks in advance.
Edit: Also, I would of course do elimination of left recursion and left factoring to make this LL(1), but first I need to figure out this main part I asked above.
Here's one that should work for your case
E = E+A | E-A | A
A = A*C | A/C | C
C = C^B | B
B = -B | D
D = (E) | id | num
As a sidenote: pay also attention to the requirements of your task since some applications might assign higher priority to the unary minus operator with respect to the power binary operator.
When writing a parser, I want to remember the location of lexemes found, so that I can report useful error messages to the programmer, as in “if-less else on line 23” or ”unexpected character on line 45, character 6” or “variable not defined” or something similar. But once I have built the syntax tree, I will transform it in several ways, optimizing or expanding some kind of macros. The transformations produce or rearrange lexemes which do not have a meaningful location.
Therefore it seems that the type representing the syntax tree should come in two flavor, a flavor with locations decorating lexemes and a flavor without lexemes. Ideally we would like to work with a purely abstract syntax tree, as defined in the OCaml book:
# type unr_op = UMINUS | NOT ;;
# type bin_op = PLUS | MINUS | MULT | DIV | MOD
| EQUAL | LESS | LESSEQ | GREAT | GREATEQ | DIFF
| AND | OR ;;
# type expression =
ExpInt of int
| ExpVar of string
| ExpStr of string
| ExpUnr of unr_op * expression
| ExpBin of expression * bin_op * expression ;;
# type command =
Rem of string
| Goto of int
| Print of expression
| Input of string
| If of expression * int
| Let of string * expression ;;
# type line = { num : int ; cmd : command } ;;
# type program = line list ;;
We should be allowed to totally forget about locations when working on that tree and have special functions to map an expression back to its location (for instance), that we could use in case of emergency.
What is the best way to define such a type in OCaml or to handle lexeme positions?
The best way is to work always with AST nodes fully annotated with the locations. For example:
type expression = {
expr_desc : expr_desc;
expr_loc : Lexing.position * Lexing.position; (* start and end *)
}
and expr_desc =
ExpInt of int
| ExpVar of string
| ExpStr of string
| ExpUnr of unr_op * expression
| ExpBin of expression * bin_op * expression
Your idea, keeping the AST free of locations and writing a function to retrieve the missing locations is not a good idea, I believe. Such a function should require searching by pointer equivalence of AST nodes or something similar, which does not really scale.
I strongly recommend to look though OCaml compiler's parser.mly which is a full scale example of AST with locations.
I'm trying to create a discriminated union for part of speech tags and other labels returned by a natural language parser.
It's common to use either strings or enums for these in C#/Java, but discriminated unions seem more appropriate in F# because these are distinct, read-only values.
In the language reference, I found that this symbol
``...``
can be used to delimit keywords/reserved words. This works for
type ArgumentType =
| A0 // subject
| A1 // indirect object
| A2 // direct object
| A3 //
| A4 //
| A5 //
| AA //
| ``AM-ADV``
However, the tags contain symbols like $, e.g.
type PosTag =
| CC // Coordinating conjunction
| CD // Cardinal Number
| DT // Determiner
| EX // Existential there
| FW // Foreign Word
| IN // Preposision or subordinating conjunction
| JJ // Adjective
| JJR // Adjective, comparative
| JJS // Adjective, superlative
| LS // List Item Marker
| MD // Modal
| NN // Noun, singular or mass
| NNP // Proper Noun, singular
| NNPS // Proper Noun, plural
| NNS // Noun, plural
| PDT // Predeterminer
| POS // Possessive Ending
| PRP // Personal Pronoun
| PRP$ //$ Possessive Pronoun
| RB // Adverb
| RBR // Adverb, comparative
| RBS // Adverb, superlative
| RP // Particle
| SYM // Symbol
| TO // to
| UH // Interjection
| VB // Verb, base form
| VBD // Verb, past tense
| VBG // Verb, gerund or persent participle
| VBN // Verb, past participle
| VBP // Verb, non-3rd person singular present
| VBZ // Verb, 3rd person singular present
| WDT // Wh-determiner
| WP // Wh-pronoun
| WP$ //$ Possessive wh-pronoun
| WRB // Wh-adverb
| ``#``
| ``$``
| ``''``
| ``(``
| ``)``
| ``,``
| ``.``
| ``:``
| `` //not sure how to escape/delimit this
``...``
isn't working for WP$ or symbols like (
Also, I have the interesting problem that the parser returns `` as a meaningful symbol, so I need to escape it as well.
Is there some other way to do this, or is this just not possible with a discriminated union?
Right now I'm getting errors like
Invalid namespace, module, type or union case name
Discriminated union cases and exception labels must be uppercase identifiers
I suppose I could somehow override toString for these goofy cases and replace the symbols with some alphanumeric equivalent?
The spec doesn't seem clear about what characters are allowed to be escaped in double-backticks in what contexts.
I think your best bet is to use standard identifiers for the DU cases, and override ToString as you suggest.
From my experience, double-backtick marks identifiers are/seem to be fully supported only in let Bindings or type members. So that means you can put about any sequence of characters inside (excepting the # character which is reserved for F# codegen).
When you want to use them as identifiers in module, type or DU cases definition, it doesn't play as nice since some characters are not supported.
E.g. ., /, *, +, $, [, ], \ or & generate an "Invalid namespace, module, type or union case name" error.