What's the difference between syntax and semantics?
I've always thought that referring to the syntax of a language was the same as referring to the semantics of a language. But I've been informed that apparently that's not the case. What's the difference?
"Colorless green ideas sleep furiously" is syntactically OK but makes no semantic sense. See http://en.wikipedia.org/wiki/Colorless_green_ideas_sleep_furiously
+1 for asking this question. I wondered the same, was too lazy to search the internet for this, and obviously never asked.
Semantics ~ Meaning
Syntax ~ Symbolic representation
So two programs written in different languages could do the same thing (semantics) but the symbols used to write the program would be different (syntax).
A compiler will check your syntax for you (compile-time errors), and derive the semantics from the language rules (mapping the syntax to machine instructions say), but won't find all the semantic errors (run-time errors, e.g. calculating the wrong result because the code says add 1 instead of add 2).
Error checking is not a criterion for distinguishing between syntax and semantics. A compiler can and must diagnose both syntax errors (like a missing semicolon) and semantic errors (like `x + y` where there's no appropriate `+` operator for those operands). Adding 1 rather than 2 is what I'd call a *logical* error.
@Keith - but logic (as in "logical error") is semantics. Some semantic checks can be done by the compiler - particularly type checking - so I agree that compilers don't only find syntax errors, but Chris only said "**won't find all** semantic errors", which doesn't imply "can't find **any**".
@Steve314: Agreed. But if you want to make a sharp distinction between errors that a compiler must detect and errors that it needn't detect, then I think "semantic" vs. "logical" is a good way to express that distinction.
@KeithThompson Actually, in theory, a compiler or interpreter for a language with a sufficiently strong and powerful (i.e., dependent) type system can check any arbitrary property of your code (modulo the Halting Problem, if applicable), so breaking semantic errors into "checkable" and "uncheckable" doesn't really make sense in general.
@Ptharien'sFlame I'm just going to pull this discussion back out of the clouds for a second by highlighting the 'in theory' part of your statement. In practice, enforcing semantics in code requires additional syntax to give the compilers cues as to the functionality. Additional semantic checking comes as a cost (ie complexity/readability). Stating that a language can be powerful enough to check all semantic errors is like saying a legal system can be perfect enough to prevent all crime. Personally, I prefer freedom over safety but that's what makes this a 'religious' topic.
Actually there are not two levels but three:
- lexical level: how characters are combined to produce language elements (
- syntactical level: how language elements are combined to produce language expressions (
)produces a conditional statement)
- semantic level: how language expressions are converted to CPU instructions in order to form a meaning (a conditional statement allows to execute one branch or the other depending on the result of the boolean expression)
A separation between lexing and parsing stages is entirely artificial, it is nothing more than an optimisation. And there are some languages where no finite flat set of lexemes is defined - but still, there is a clearly defined syntax. So, I'd prefer to define lexemes as part of a syntax, is is not a separate entity.
@SK-logic: In many languages, the list of authorised or forbidden lexemes forming a variable name is specified. So the separation makes sense.
@mouviciel, it make sense as an optimisation only - otherwise you'll just have a `ValidIdentifier` terminal, which could be defined as something like `![AnyKeyword] [Identifier]` (I'm using PEG-like notation here). You don't need a separate lexing pass for such a language. See, for example, GLR-based C++ parsers.
@SK-logic Only in purely context-free languages is the parser unnecessary. As soon as another level of context is added the parser becomes necessary. For example access levels (private/public/protected), inheritance, preprocessing, etc all require additional context to determine the semantics - therefore the code that implements them is broken/useless after performing just the lexer stage. Some code may be available in a pre-parsed state (ie dlls, bytecode) but it had to go through the lexer/parser/compiler stage at some point.
@EvanPlaice, what are you talking about? My point is that *lexing* is not necessary (and actually limits your language), not *parsing*.
@SK-logic I guess I read your comment to mean the opposite of what you intended. I thought you were talking about cases where only a lexer is needed - like in purely 'regular' or 'context-free' languages. In higher level languages a lexer may not be necessary but it provides a quick way to run a single pass syntax validation. I completely agree that there are many cases where it would be beneficial to turn off or completely eliminate the lexer stage.
- lexical level: how characters are combined to produce language elements (
Semantics describe the logical entities of a programming language and their interactions. Syntax defines how these are expressed in characters.
For example, the concept of pointer arithmetic is part of C's semantics; the way the
-operators can be used to express pointer operations are part of its syntax.
So, "Paradigms" are related with semantics? I mean a paradigm is a set of interrelated semantics?
You did not specify whether you only refer to programming languages or to general languages used in programming, so my answer is about data languages (such as XML, RDF, data type systems etc.):
Brian L. Meek in his seven golden rules for producing language-independent standards (1995) writes that "one language's syntax can be another's semantics". He refers to the words "syntax" and "semantic" used in data description: so if you stumble upon these words in a specification of some data format, you should better replace both words with "Potrzebie" to make clear that you must work out the meaning for yourself.
The relation between syntax and semantic, at least in exactly specified data, can better be described by the term "encoding". Semantic is encoded in syntax. As recordings can be nested, one language's syntax is another's semantics. If one goes beyond the realm of data, this nesting can be virtually infinite, as described by Umberto Eco as "unlimited semiosis".
To give a an example:
- XML syntax (the stuff with all these brackets) is syntax with an XML Infoset (an abstract tree) as semantic.
- An XML Infoset as syntax can express a record in some XML data format as semantic, for instance an RDF/XML document that encodes an RDF graph.
- An RDF graph (the stuff with URI References) as syntax encodes a graph of abstract resources as semantic.
- A graph of abstract resources as syntax encodes a conceptual model as semantic.
People usually stop at some level and take it as semantic, but in the end there is no final semantic unless some human being interprets the data in his mind. As soon as one tries to express semantic in form of data, it becomes syntax.
If it can be described in BNF (Backus-Naur Form) or something similar, it's syntax. If it can't, it's not.
Semantics, on the other hand, is about the meaning of a program (or other chunk of source code).
And sometimes the line between the two can be blurry.
One way to understand the distinction is to look at the kinds of errors you get when your program's syntax or semantics is incorrect.
A syntax error is a failure of the source code to match the language grammar, for example, not having a semicolon where one is required.
A semantic error is a failure to satisfy other language requirements (what C, for example, calls "constraints"); an example might be writing
x + ywhere
yare of incompatible types. The language grammar tells you that an addition looks like
something + something, but it's not powerful enough to express the requirements on the types of the left and right operands.
(Logical errors, such as using 1 where 2 would be correct, are not generally detectable by the compiler -- though in some cases a compiler can warn about questionable code.)
Syntax is what the (lexical) symbols say. Semantics is what they mean.
condition ? true_value : false_value
If(condition, true_value, false_value)
-- Different syntax, same semantics.
left_value / right_value
left_value / right_value
-- Same syntax, different semantics (for integers).
Syntax is grammatical arrangement of words in a sentence i.e. word order.
(English) ‘cat dog boy’ and (programming) ‘hi.5’ is not syntactically correct.
(English) ‘cat hugs boy’ and (programming) ‘*3.2*5*’ is syntactically valid.
Static Semantics is whether syntactically valid statements have any meaning.
(English) ‘I are big’ (programming)(python) ‘3 + ‘hi’’ is syntactically correct but has static semantic error.
Semantics is the meaning associated with syntactically correct string of symbols with no static semantic error i.e. sentence is syntactically and semantically correct, but its meaning may not be what was intended.
(English) ‘Flying planes can be dangerous’ can have two meanings i.e. flying of planes can be dangerous or the planes that are flying can be dangerous.
(Programming) ‘the computer will not generate any error messages, but it will not do what you told it to do; it will do something else.’
Source: MIT 6.00.1
Syntax and semantics is like strategy and tactics or left and right.
They are not really independent universal concepts, but a related pair of words that, when you are in a particular context, indicate opposite directions. But the same thing that is strategy on one scale is tactics on another.
So if you are writing code in a language, the syntax is the language you are using and the desired behaviour is the semantics. But if you are implementing, or discussing, the compiler for that language, then the syntax is the grammar and perhaps type system and the semantics everything built on that. And so on.