What is the difference between syntax and semantics in programming languages (like C, C++)?
TL; DR
In summary, syntax is the concept that concerns itself only whether or not the sentence is valid for the grammar of the language. Semantics is about whether or not the sentence has a valid meaning.
Long answer:
Syntax is about the structure or the grammar of the language. It answers the question: how do I construct a valid sentence? All languages, even English and other human (aka "natural") languages have grammars, that is, rules that define whether or not the sentence is properly constructed.
Here are some C language syntax rules:
separate statements with a semi-colon
enclose the conditional expression of an IF statement inside parentheses
group multiple statements into a single statement by enclosing in curly braces
data types and variables must be declared before the first executable statement (this feature has been dropped in C99. C99 and latter allow mixed type declarations.)
Semantics is about the meaning of the sentence. It answers the questions: is this sentence valid? If so, what does the sentence mean? For example:
x++; // increment
foo(xyz, --b, &qrs); // call foo
are syntactically valid C statements. But what do they mean? Is it even valid to attempt to transform these statements into an executable sequence of instructions? These questions are at the heart of semantics.
Consider the ++ operator in the first statement. First of all, is it even valid to attempt this?
If x is a float data type, this statement has no meaning (according to the C language rules) and thus it is an error even though the statement is syntactically correct.
If x is a pointer to some data type, the meaning of the statement is to "add sizeof(some data type) to the value at address x and store the result into the location at address x".
If x is a scalar, the meaning of the statement is "add one to the value at address x and store the result into the location at address x".
Finally, note that some semantics can not be determined at compile-time and therefore must be evaluated at run-time. In the ++ operator example, if x is already at the maximum value for its data type, what happens when you try to add 1 to it? Another example: what happens if your program attempts to dereference a pointer whose value is NULL?
Syntax refers to the structure of a language, tracing its etymology to how things are put together.
For example you might require the code to be put together by declaring a type then a name and then a semicolon, to be syntactically correct.
Type token;
On the other hand, the semantics is about meaning.
A compiler or interpreter could complain about syntax errors. Your co-workers will complain about semantics.
Semantics is what your code means--what you might describe in pseudo-code. Syntax is the actual structure--everything from variable names to semi-colons.
Wikipedia has the answer. Read syntax (programming languages) & semantics (computer science) wikipages.
Or think about the work of any compiler or interpreter. The first step is lexical analysis where tokens are generated by dividing string into lexemes then parsing, which build some abstract syntax tree (which is a representation of syntax). The next steps involves transforming or evaluating these AST (semantics).
Also, observe that if you defined a variant of C where every keyword was transformed into its French equivalent (so if becoming si, do becoming faire, else becoming sinon etc etc...) you would definitely change the syntax of your language, but you won't change much the semantics: programming in that French-C won't be easier!
You need correct syntax to compile.
You need correct semantics to make it work.
Late to the party - but to me, the answers here seem correct but incomplete.
Pragmatically, I would distinguish between three levels:
Syntax
Low level semantics
High level semantics
1. SYNTAX
Syntax is the formal grammar of the language, which specifies a well-formed statement the compiler will recognise.
So in C, the syntax of variable initialisation is:
data_type variable_name = value_expression;
Example:
int volume = 66 * 22 * 55;
While in Go, which offers type inference, one form of initialisation is:
variable_name := value_expression
Example:
volume := 66 * 22 * 55
Clearly, a Go compiler won't recognise the C syntax, and vice versa.
2. LOW LEVEL SEMANTICS
Where syntax is concerned with form, semantics is concerned with meaning.
In natural languages, a sentence can be syntactically correct but semantically meaningless. For example:
The man bought the infinity from the store.
The sentence is grammatically correct but doesn't make real-world sense.
At the low level, programming semantics is concerned with whether a statement with correct syntax is also consistent with the semantic rules as expressed by the developer using the type system of the language.
For example, this is a syntactically correct assignment statement in Java, but semantically it's an error as it tries to assign an int to a String
String firstName = 23;
So type systems are intended to protect the developer from unintended slips of meaning at the low level.
Loosely typed languages like JavaScript or Python provide very little semantic protection, while languages like Haskell or F# with expressive type systems provide the skilled developer with a much higher level of protection.
For example, in F# your ShoppingCart type can specify that the cart must be in one of three states:
type ShoppingCart =
| EmptyCart // no data
| ActiveCart of ActiveCartData
| PaidCart of PaidCartData
Now the compiler can check that your code hasn't tried to put the cart into an illegal state.
In Python, you would have to write your own code to check for valid state.
3. HIGH LEVEL SEMANTICS
Finally, at a higher level, semantics is concerned with what the code is intended to achieve - the reason that the program is being written.
This can be expressed as pseudo-code which could be implemented in any complete language. For example:
// Check for an open trade for EURUSD
// For any open trade, close if the profit target is reached
// If there is no open trade for EURUSD, check for an entry signal
// For an entry signal, use risk settings to calculate trade size
// Submit the order.
In this (heroically simplified) scenario, you are making a high-level semantic error if your system enters two trades at once for EURUSD, enters a trade in the wrong direction, miscalculates the trade size, and so on.
TL; DR
If you screw up your syntax or low-level semantics, your compiler will complain.
If you screw up your high-level semantics, your program isn't fit for purpose and your customer will complain.
Syntax is the structure or form of expressions, statements, and program units but Semantics is the meaning of those expressions, statements, and program units. Semantics follow directly from syntax.
Syntax refers to the structure/form of the code that a specific programming language specifies but Semantics deal with the meaning assigned to the symbols, characters and words.
Understanding how the compiler sees the code
Usually, syntax and semantics analysis of the code is done in the 'frontend' part of the compiler.
Syntax: Compiler generates tokens for each keyword and symbols: the token contains the information- type of keyword and its location in the code.
Using these tokens, an AST(short for Abstract Syntax Tree) is created and analysed.
What compiler actually checks here is whether the code is lexically meaningful i.e. does the 'sequence of keywords' comply with the language rules? As suggested in previous answers, you can see it as the grammar of the language(not the sense/meaning of the code).
Side note: Syntax errors are reported in this phase.(returns tokens with the error type to the system)
Semantics: Now, the compiler will check whether your code operations 'makes sense'.
e.g. If the language supports Type Inference, sematic error will be reported if you're trying to assign a string to a float. OR declaring the same variable twice.
These are errors that are 'grammatically'/ syntaxially correct, but makes no sense during the operation.
Side note: For checking whether the same variable is declared twice, compiler manages a symbol table
So, the output of these 2 frontend phases is an annotated AST(with data types) and symbol table.
Understanding it in a less technical way
Considering the normal language we use; here, English:
e.g. He go to the school. - Incorrect grammar/syntax, though he wanted to convey a correct sense/semantic.
e.g. He goes to the cold. - cold is an adjective. In English, we might say this doesn't comply with grammar, but it actually is the closest example to incorrect semantic with correct syntax I could think of.
He drinks rice (wrong semantic- meaningless, right syntax- grammar)
Hi drink water (right semantic- has meaning, wrong syntax- grammar)
Syntax: It is referring to grammatically structure of the language.. If you are writing the c language . You have to very care to use of data types, tokens [ it can be literal or symbol like "printf()". It has 3 tokes, "printf, (, )" ]. In the same way, you have to very careful, how you use function, function syntax, function declaration, definition, initialization and calling of it.
While semantics, It concern to logic or concept of sentence or statements. If you saying or writing something out of concept or logic, then you are semantically wrong.
Related
I am learning about compilers, specifically looking at 2-phase compilers, and am confused on the certain phases where errors are detected. Let's say we have something like:
int x, y;
x = x + y + z;
Where we are trying to reference a variable that has not been declared. I think this is an error that would be detected in the front-end of the compiler. But I don't know which sub-area of the front-end would detect this error.
The three parts to the front-end are: the scanner, the parser, and the context-free analyzer. The scanner reads every single character in a statement and splits the statement up into tokens. So, I could be wrong, but I don't think the error would be detected here. The parser checks to see if the statement is syntactically correct. Here is where I start to get confused. Even though z is undeclared, the syntax of the statement is technically correct, so would the error not be detected here either? The context-free analyzer uses the symbol table and syntax tree to check the program to see if it is semantically consistent with the language definition. Here it also does type checking. Would it be here that the error would be detected? Because at this point the compiler would look in the symbol table and notice that z doesn't have a type (or that it's not in there at all?). Or is this something that would be detected by the back-end of the compiler? If it is the back-end, I don't understand why that is the case. Any clarification would be highly appreciated. Thanks.
This is ultimately compiler-dependent, but typically this would come up at the semantic analysis level, which is still in the compiler front-end.
With a traditional compiler, this couldn't be done in the scanning phase because scanners use finite automata and the language of "strings that represent proper variable scoping" isn't regular. This also typically wouldn't be done as part of parsing, since parsing usually is about building up an AST and, if it were done bottom-up, the scoping information wouldn't be available at the time that the parser determined the structure of the code.
However, the semantic analyzer has all the information necessary to find this error - it has the AST and can use that to build a symbol table, walk through all the expressions in the code, and notice that z isn't anywhere in that symbol table.
Whenever I see a Julia macro in use like #assert or #time I'm always wondering about the need to distinguish a macro syntactically with the # prefix. What should I be thinking of when using # for a macro? For me it adds noise and distraction to an otherwise very nice language (syntactically speaking).
I mean, for me '#' has a meaning of reference, i.e. a location like a domain or address. In the location sense # does not have a meaning for macros other than that it is a different compilation step.
The # should be seen as a warning sign which indicates that the normal rules of the language might not apply. E.g., a function call
f(x)
will never modify the value of the variable x in the calling context, but a macro invocation
#mymacro x
(or #mymacro f(x) for that matter) very well might.
Another reason is that macros in Julia are not based on textual substitution as in C, but substitution in the abstract syntax tree (which is much more powerful and avoids the unexpected consequences that textual substitution macros are notorious for).
Macros have special syntax in Julia, and since they are expanded after parse time, the parser also needs an unambiguous way to recognise them
(without knowing which macros have been defined in the current scope).
ASCII characters are a precious resource in the design of most programming languages, Julia very much included. I would guess that the choice of # mostly comes down to the fact that it was not needed for something more important, and that it stands out pretty well.
Symbols always need to be interpreted within the context they are used. Having multiple meanings for symbols, across contexts, is not new and will probably never go away. For example, no one should expect #include in a C program to go viral on Twitter.
Julia's Documentation entry Hold up: why macros? explains pretty well some of the things you might keep in mind while writing and/or using macros.
Here are a few snippets:
Macros are necessary because they execute when code is parsed,
therefore, macros allow the programmer to generate and include
fragments of customized code before the full program is run.
...
It is important to emphasize that macros receive their arguments as
expressions, literals, or symbols.
So, if a macro is called with an expression, it gets the whole expression, not just the result.
...
In place of the written syntax, the macro call is expanded at parse
time to its returned result.
It actually fits quite nicely with the semantics of the # symbol on its own.
If we look up the Wikipedia entry for 'At symbol' we find that it is often used as a replacement for the preposition 'at' (yes it even reads 'at'). And the preposition 'at' is used to express a spatial or temporal relation.
Because of that we can use the #-symbol as an abbreviation for the preposition at to refer to a spatial relation, i.e. a location like #tony's bar, #france, etc., to some memory location #0x50FA2C (e.g. for pointers/addresses), to the receiver of a message (#user0851 which twitter and other forums use, etc.) but as well for a temporal relation, i.e. #05:00 am, #midnight, #compile_time or #parse_time.
And since macros are processed at parse time (here you have it) and this is totally distinct from the other code that is evaluated at run time (yes there are many different phases in between but that's not the point here).
In addition to explicitly direct the attention to the programmer that the following code fragment is processed at parse time! as oppossed to run time, we use #.
For me this explanation fits nicely in the language.
thanks#all ;)
Obviously, it is a convention in Racket/Scheme to append an exclamation point to function names that perform mutation. For example, in Racket, set!, box-set!, vector-set!, etc. Certain functions have side-effects, like print, but since those side-effects are "harmless," I understand why they don't usually come with exclamation marks attached.
However, this convention is arbitrarily violated. For example, async-channel-get and async-channel-put clearly perform mutation, but they don't have the "mutation marker" appended to their names. This can be somewhat justified by pointing out that these are channels, clearly mutation-based, so the "!" would be superfluous.
This is not a valid justification for everything, though. Racket's WebSockets library provides ws-send! and ws-close! functions, both with the obvious markers, but ws-recv does not! Is this just an isolated violation of the convention, or is there some rule?
I ask this mostly to be sure of how I should name functions in my own code. When should I use the exclamation mark, when should I not? I recognize that it's just a convention, not a rule, and it will likely be somewhat inconsistent, but I still would like to know what the best practices are.
I don't think #!racket has its own naming convention, but according to this R5RS page you can read:
The names of procedures and syntactic forms that cause side effects
end with an exclamation point ( ! ). These include set! and
vector-set!. Procedures that perform input or output technically cause
side effects, but their names are exceptions to this rule.
In the Scheme wiki variable naming convention it says procedure! is for "significant side effects". IMO that means the side effects is the hero of the procedure as in set-car! and set! while read returns a value which perhaps is the main feature of read?
I am confused by the terms for a long time, thinking it is good to ask out what exactly do they mean:
A. syntax. B. syntax value. C. syntax object. D.s-expression E.datum (in syntax->datum)
What's the difference between s-expression and symbol?
What's the difference between s-expression and datum?
What's the difference between (syntax, syntax values and syntax object) from s-expression?
Code examples for explanation will be appreciated.
"Syntax" is a type for representing source code in Racket, which is a wrapper around S-expression (see a recent blog post for details). "Syntax value" and "syntax object" are all synonyms of this, and ni the ancient days of the mzscheme language functions that deal with syntax used syntax-value in the name. These days we use just "syntax" more often, and for a plural form we use "syntaxes".
An "S-expression" is either a primitive piece of data that can be typed in code (symbols, numbers, strings, booleans, etc -- in Racket you could also include other types), or a list of these things. An S-expression is therefore any nested structure of lists made of these primitive types at the fringe. Sometimes this includes vectors too (since they can be typed in using the #(...) syntax) but more usually they're left out.
Finally, "datum" is another name for an S-expression, sometimes when you want to refer to the fact that it's a piece of data that has an input representation. You can see how R5RS introduces it: <Datum> may be any external representation of a Scheme object [...]. This notation is used to include literal constants in Scheme code.
As for your questions:
What's the difference between s-expression and symbol?
A symbols is an S-expression, an S-expression may contain symbols.
What's the difference between s-expression and datum?
Nothing really. (Although some subtle intentions differences might be there.)
What's the difference between (syntax, syntax values and syntax object) from s-expression?
They are the representation of program syntax used by macros in racket -- they contain the S-expressions, but they add source location information, lexical context, syntax properties, and certificates. See that blog post for a quick introduction.
for our compiler theory class, we are tasked with creating a simple interpreter for our own designed programming language. I am using jflex and cup as my generators but i'm a bit stuck with what a lexical error is. Also, is it recommended that i use the state feature of jflex? it feels wrong as it seems like the parser is better suited to handling that aspect. and do you recommend any other tools to create the language. I'm sorry if i'm impatient but it's due on tuesday.
A lexical error is any input that can be rejected by the lexer. This generally results from token recognition falling off the end of the rules you've defined. For example (in no particular syntax):
[0-9]+ ===> NUMBER token
[a-zA-Z] ===> LETTERS token
anything else ===> error!
If you think about a lexer as a finite state machine that accepts valid input strings, then errors are going to be any input strings that do not result in that finite state machine reaching an accepting state.
The rest of your question was rather unclear to me. If you already have some tools you are using, then perhaps you're best to learn how to achieve what you want to achieve using those tools (I have no experience with either of the tools you mentioned).
EDIT: Having re-read your question, there's a second part I can answer. It is possible that a language could have no lexical errors - it's the language in which any input string at all is valid input.
A lexical error could be an invalid or unacceptable character by the language, like '#' which is rejected as a lexical error for identifiers in Java (it's reserved).
Lexical errors are the errors thrown by your lexer when unable to continue. Which means that there's no way to recognise a lexeme as a valid token for you lexer. Syntax errors, on the other side, will be thrown by your scanner when a given set of already recognised valid tokens don't match any of the right sides of your grammar rules.
it feels wrong as it seems like the
parser is better suited to handling
that aspect
No. It seems because context-free languages include regular languages (meaning than a parser can do the work of a lexer). But consider than a parser is a stack automata, and you will be employing extra computer resources (the stack) to recognise something that doesn't require a stack to be recognised (a regular expression). That would be a suboptimal solution.
NOTE: by regular expression, I mean... regular expression in the Chomsky Hierarchy sense, not a java.util.regex.* class.
lexical error is when the input doesn't belong to any of these lists:
key words: "if", "else", "main"...
symbols: '=','+',';'...
double symbols: ">=", "<=", "!=", "++"
variables: [a-z/A-Z]+[0-9]*
numbers: [0-9]*
examples: 9var: error, number before characters, not a variable and not a key word either.
$: error
what I don't know is whether something like more than one symbol after each other is accepted, like "+-"
Compiler can catch an error when it has the grammar in it!
It will depend on the compiler itself whether it has the capacity (scope) of catching the lexical errors or not.
If is decided during the development of compiler what types of lexical error and how (according to the grammar) they are going to be handled.
Usually all famous and mostly used compiler has this capabilities.