Is such type of an error produced during type checking or when input is being parsed?
Under what type should the error be addressed?
The way I see it it is a semantic error, because your language parses just fine even though your are using an identifier which you haven't previously bound--i.e. syntactic analysis only checks the program for well-formed-ness. Semantic analysis actually checks that your program has a valid meaning--e.g. bindings, scoping or typing. As #pst said you can do scope checking during parsing, but this is an implementation detail. AFAIK old compilers used to do this to save some time and space, but I think today such an approach is questionable if you don't have some hard performance/memory constraints.
The program conforms to the language grammar, so it is syntactically correct. A language grammar doesn't contain any statements like 'the identifier must be declared', and indeed doesn't have any way of doing so. An attempt to build a two-level grammar along these lines failed spectacularly in the Algol-68 project, and it has not been attempted since to my knowledge.
The meaning, if any, of each is a semantic issue. Frank deRemer called issues like this 'static semantics'.
In my opinion, this is not strictly a syntax error - nor a semantic one. If I were to implement this for a statically typed, compiled language (like C or C++), then I would not put the check into the parser (because the parser is practically incapable of checking for this mistake), rather into the code generator (the part of the compiler that walks the abstract syntax tree and turns it into assembly code). So in my opinion, it lies between syntax and semantic errors: it's a syntax-related error that can only be checked by performing semantic analysis on the code.
If we consider a primitive scripting language however, where the AST is directly executed (without compilation to bytecode and without JIT), then it's the evaluator/executor function itself that walks the AST and finds the undeclared variable - in this case, it will be a runtime error. The difference lies between the "AST_walk()" routine being in different parts of the program lifecycle (compilation time and runtime), should the language be a scripting or a compiled one.
In the case of languages -- and there are many -- which require identifiers to be declared, a program with undeclared identifiers is ill-formed and thus a missing declaration is clearly a syntax error.
The usual way to deal with this is to incorporate information about symbols in a symbol table, so that the parse can use this information.
Here are a few examples of how identifier type affects parsing:
C / C++
A classic case:
(a)-b;
Depending on a, that's either a cast or a subtraction:
#include <stdio.h>
#if TYPEDEF
typedef double a;
#else
double a = 3.0;
#endif
int main() {
int b = 3;
printf("%g\n", (a)-b);
return 0;
}
Consequently, if a hadn't been declared at all, the compiler must reject the program as syntactically ill-formed (and that is precisely the word the standard uses.)
XML
This one is simple:
<block>Hello, world</blob>
That's ill-formed XML, but it cannot be detected with a CFG. (Nonetheless, all XML parsers will correctly reject it as ill-formed.) In the case of HTML/SGML, where end-tags may be omitted under some well-defined circumstances, parsing is trickier but nonetheless deterministic; again, the precise declaration of a tag will determine the parse of a valid input, and it's easy to come up with inputs which parse differently depending on declaration.
English
OK, not a programming language. I have lots of other programming language examples, but I thought this one might trigger some other intuitions.
Consider the two grammatically correct sentences:
The sheep is in the meadow.
The sheep are in the meadow.
Now, what about:
The cow is in the meadow.
(*) The cow are in the meadow.
The second sentence is intelligible, albeit ambiguous (is the noun or the verb wrong?) but it is certainly not grammatically correct. But in order to know that (and other similar examples), we have to know that sheep has an unmarked plural. Indeed, many animals have unmarked plurals, so I recognize all the following as grammatical:
The caribou are in the meadow.
The antelope are in the meadow.
The buffalo are in the meadow.
But definitely not:
(*) The mouse are in the meadow.
(*) The bird are in the meadow.
etc.
It seems that there is a common misconception that because the syntactic analyzer uses a context free grammar parser, that syntactic analysis is restricted to parsing a context free grammar. This is simply not true.
In the case of C (and family), the syntax analyzer uses a symbol table to help it parse. In the case of XML, it uses the tag stack, and in the case of generalize SGML (including HTML) it also uses tag declarations. Consequently, the syntax analyzer considered as a whole is more powerful than the CFG, which is just a part of the analysis.
The fact that a given program passes the syntax analysis does not mean that it is semantically correct. For example, the syntax analyser needs to know whether a is a type or not in order to correctly parse (a)-b, but it does not need to know whether the cast is in fact possible, in the case that it a is a type, or that a and b can meaningfully be subtracted, in the case that a is a variable. These verifications can happen during type analysis after the parse tree is built, but they are still compile-time errors.
Related
I am playing around with Tatsu to implement a parser for a language used in the semiconductor industry. This language requires that variables be defined before usage. So for example:
SignalGroup { A: In; B: Out};
Pattern {
V {A=1, B=1 }
V {A=1, B=0 }
};
In this case, the SignalGroup block must come before the Pattern block. How do I enforce/implement this "ordering" when writing the grammer in TatSu?
Although for some languages it is possible to write grammars that verify if the same symbol appears on different places, the grammars usually end up being too complicated to be useful.
Compilers (translators) are usually implemented with separate lexical, syntactical, and semantic analyzer components. There are several reasons for that:
Each component is so well focused that it is clearer and easier to write.
Each component is very efficient
The most common errors (which are exactly lexical, syntactical, and semantic) can be reported earlier
With those components in mind, checking if a symbol has ben previously defined belongs to the semantic (meaning) aspect of the program, and the way to check is to keep a symbol table that is filled when the definition parts of the input are being parsed, and queried on the use parts of the input are being parsed.
In TatSu in particular the different components are well separated, yet run in parallel. For your requirement you just need to use the simplest grammar that allows for the semantic actions that store and query the symbols. By raising FailedSemantics from within semantic actions, any semantic errors will be reported exactly as the lexical and syntactical ones so the user doesn't have to think about which component flagged each error.
If you use the Python parser generation in TatSu, the translator will generate the skeleton of a semantic actions class as part of the output.
Context
I've recently come up with an issue that I couldn't solve by myself in a parser I'm writing.
This parser is a component in a compiler I'm building and the question is in regards to the expression parsing necessary in programming language parsing.
My parser uses recursive descent to parse expressions.
The problem
I parse expressions using normal regular language parsing rules, I've eliminated left recursion in all my rules but there is one syntactic "ambiguity" which my parser simply can't handle and it involves generics.
comparison → addition ( ( ">" | ">=" | "<" | "<=" ) addition )* ;
is the rule I use for parsing comparison nodes in the expression
On the other hand I decided to parse generic expressions this way:
generic → primary ( "<" arguments ">" ) ;
where
arguments → expression ( "," expression )* ;
Now because generic expressions have higher precedence as they are language constructs and not mathematical expressions, it causes a scenario where the generic parser will attempt to parse expressions when it shouldn't.
For example in a<2 it will parse "a" as a primary element of the identifier type, immediately afterwards find the syntax for a generic type, parse that and fail as it can't find the closing tag.
What is the solution to such a scenario? Especially in languages like C++ where generics can also have expressions in them if I'm not mistaken arr<1<2> might be legal syntax.
Is this a special edge case or does it require a modification to the syntax definition that im not aware of?
Thank you
for example in a<2 it will parse "a" as a primary element of the identifier type, immideatly afterwards find the syntax for a generic type, parse that and fail as it cant find the closing tag
This particular case could be solved with backtracking or unbounded lookahead. As you said, the parser will eventually fail when interpreting this as a generic, so when that happens, you can go back and parse it as a relational operator instead. The lookahead variant would be to look ahead when seeing a < to check whether the < is followed by comma-separated type names and a > and only go into the generic rule if that is the case.
However that approach no longer works if both interpretations are syntactically valid (meaning the syntax actually is ambiguous). One example of that would be x<y>z, which could either be a declaration of a variable z of type x<y> or two comparisons. This example is somewhat unproblematic since the latter meaning is almost never the intended one, so it's okay to always interpret it as the former (this happens in C# for example).
Now if we allow expressions, it becomes more complicated. For x<y>z it's easy enough to say that this should never be interpreted as two comparison as it makes no sense to compare the result of a comparison with something else (in many languages using relational operators on Booleans is a type error anyway). But for something like a<b<c>() there are two interpretations that might both be valid: Either a is a generic function called with the generic argument b<c or b is a generic function with the generic argument c (and a is compared to the result of calling that function). At this point it is no longer possible to resolve that ambiguity with syntactic rules alone:
In order to support this, you'll need to either check whether the given primary refers to a generic function and make different parsing decisions based on that or have your parser generate multiple trees in case of ambiguities and then select the correct one in a later phase. The former option means that your parser needs to keep track of which generic functions are currently defined (and in scope) and then only go into the generic rule if the given primary is the name of one of those functions. Note that this becomes a lot more complicated if you allow functions to be defined after they are used.
So in summary supporting expressions as generic arguments requires you to keep track of which functions are in scope while parsing and use that information to make your parsing decisions (meaning your parser is context sensitive) or generate multiple possible ASTs. Without expressions you can keep it context free and unambiguous, but will require backtracking or arbitrary lookahead (meaning it's LL(*)).
Since neither of those are ideal, some languages change the syntax for calling generic functions with explicit type parameters to make it LL(1). For example:
Java puts the generic argument list of a method before the method name, i.e. obj.<T>foo() instead of obj.foo<T>().
Rust requires :: before the generic argument list: foo::<T>() instead of foo<T>().
Scala uses square brackets for generics and for nothing else (array subscripts use parentheses): foo[T]() instead of foo<T>().
I am wondering how the syntax analysis and semantic analysis work.
I have finished the lexer and the grammar construction of my interpreter.
Now I am going to implement a recursive descent (top down) parser for this grammar
For example, I have the following grammar:
<declaration> ::= <data_type> <identifier> ASSIGN <value>
so i coded it like this (in java):
public void declaration(){
data_type();
identifier();
if(token.equals("ASSIGN")){
lexer(); //calls next token
value();
} else {
error();
}
}
Assuming I have three data types: Int, String and Boolean. Since the values for each data types are different, (ex. true or false only in Boolean) how can I determine if it fits the data type correctly? What part of my code would determine that?
I am wondering where would I put the code to:
1.) call the semantic analysis part of my program.
2.) store my variables into the symbol table.
Do syntax analysis and semantic analysis happen at the same time?
or do i need to finish the syntax analysis first, then do the semantic analysis?
I am really confused. Please help.
Thank you.
You can do syntax analysis (parsing) and semantic analysis (e.g., checking for agreement between <data_type> and <value>) at the same time. For example, when declaration() calls data_type(), the latter could return something (call it DT) that indicates whether the declared type is Int, String or Boolean. Similarly, value() could return something (VT) that indicates the type of the parsed. Then declaration() would simply compare DT and VT, and raise an error if they don't match. (Alternatively, value() could take a parameter indicating the declared type, and it could do the check.)
However, you may find it easier to completely separate the two phases. To do so, you would typically have the parsing phase build a parse tree (or abstract syntax tree). So your top level would call (e.g.) program() to parse a whole program, which would return a tree representing (the syntax of) the program, and you would pass that tree to a semantic_analysis() routine, which would traverse the tree, extracting pertinent information and enforcing semantic constraints.
The short answer is: it depends on the definition of your programming language. And, since you only specified one derivation rule and three native types, there is no way of knowing. For example, if your programming language allows forward declarations like the c++ code below, then handling the derivation rule for function declaration (foo) is done without knowing the type of the variable serial
class Tree {
public:
int foo(void)
{
return serial;
}
int serial;
};
Indeed, modern compilers separate the syntax analysis phase from the semantic analysis phase. The syntax analysis phase is performed first, making sure the input program agrees with the context free grammar of the language. And, in addition, produces an Abstract Syntax Tree (AST). Note the difference between an AST and a parse tree, as discussed in this SO post. The semantic analysis phase then traverses the AST and checks for type mismatches among other things.
Having said that, toy programming languages can sometimes couple semantic and syntax analysis together. When a recursive descent parser is used,
you should have relevant recursive calls return a type.
By concept/function/implementation, what are the differences between compilers and parsers?
A compiler is often made up of several components, one of which is a parser.
A common set of components in a compiler is:
Lexer - break the program up into words.
Parser - check that the syntax of the sentences are correct.
Semantic Analysis - check that the sentences make sense.
Optimizer - edit the sentences for brevity.
Code generator - output something with equivalent semantic meaning using another vocabulary.
To add a little bit:
As mentioned elsewhere, small C is a recursive decent compiler that generated code as it parsed. Basically syntactical analysis, semantic analysis, and code generation in one pass. As I recall, it also lexed in the parser.
A long time ago, I wrote a C compiler (actually several: the Introl-C family for microcontrollers) that used recursive descent and did syntax and semantic checking during the parse and produced a tree representation of the program from which code was generated.
Today, I'm working on a compiler that does source -> tokens -> AST -> IR -> code, pretty much as I described above.
A parser just reads a text into an internal, more abstract representation, often a tree or graph of some sort.
A compiler translates such an internal representation into another format. Most often this means converting source code into executable programs. But the target doesn't have to be machine code. It can be another programming language as well; the compiler would still be a compiler. Obviously a compiler needs a parser to actually read its input.
Compiler always have a parser inside. Parser just process the language and return the tree representation of it, compiler generate something from that tree, actual machine codes or another language.
A parser is one element of a compiler.
Are you looking for the differences between an interpreter and a compiler?
A parser takes in raw-data and parses it into a tree structure. This syntax-tree is then passed on to generator, which will turn it into whatever it is supposed to generate.
So, a parser is a part of a compiler.
In general, parser is a part of the compiler, but compiler is designed to convert the received script generally into machine-readable code or sometimes into another language.
A compiler is a special type of computer program that translates a human readable text file into a form that the computer can more easily understand. At its most basic level, a computer can only understand two things, a 1 and a 0. At this level, a human will operate very slowly and find the information contained in the long string of 1s and 0s incomprehensible. A compiler is a computer program that bridges this gap.
A parser is a piece of software that evaluates the syntax of a script when it is executed on a web server. For scripting languages used on the web, the parser works like a compiler might work in other types of application development environments.Parsers are commonly used in script development because they can evaluate code when the script is executed and do not require that the code be compiled first.
Given a grammar and the attached action code, are there any standard solution for deducing what type each production needs to result in (and consequently, what type the invoking production should expect to get from it)?
I'm thinking of an OO program and action code that employs something like c#'s var syntax (but I'm not looking for something that is c# specific).
This would be fairly simple if it were not for function overloading and recursive grammars.
The issue arises with cases like this:
Foo ::=
Bar Baz { return Fig(Bar, Baz); }
Foo Pit { return Pop(Foo, Pit); } // typeof(foo) = fn(typeof(Foo))
If you are writing code in a functional language it is easy; standard Hindley-Milner type inference works great. Do not do this. In my EBNF parser generator (never released but source code available on request), which supports Icon, c, and Standard ML, I actually implemented the idea you are asking about for the Standard ML back end: all the types were inferred. The resulting grammars were nearly impossible to debug.
If you throw overloading into the mix, the results are only going to be harder to debug. (That's right! This just in! Harder than impossible! Greater than infinity! Past my bedtime!) If you really want to try it yourself you're welcome to my code. (You don't; there's a reason I never released it.)
The return value of a grammar action is really no different from a local variable, so you should be able to use C# type inference to do the job. See this paper for some insight into how C# type inference is implemented.
The standard way of doing type inference is the Hindley-Milner algorithm, but that will not handle overloading out-of-the-box.
Note that even parser generators for type-inferencing languages don't typically infer the types of grammar actions. For example, ocamlyacc requires type annotations. The Happy parser generator for Haskell can infer types, but seems to discourage the practice. This might indicate that inferring types in grammars is difficult, a bad idea, or both.
[UPDATE] Very much pwned by Norman Ramsey, who has the benefit of bitter experience.