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?
Related
Isn’t list a keyword to create a new list in Lisp, but yet it is possible to have an argument called list in Lisp. I thought keywords in most programming languages such as Java or C++ cannot be used for argument names, is there a special reason in Lisp that they can?
The name list isn't a reserved keyword, it's an ordinary function. Reusing the name for another purpose can be confusing for the reader but doesn't present any problems for the language itself; it's the same as having two variables called x in different parts of the program.
Mainstream Lisp descendants and derivatives like Commmon Lisp and Scheme do not incorporate the concept of reserved keywords. It is alien to the way Lisp works.
When Lisp read syntax is scanned, identifier tokens which appear in it are converted into corresponding symbol objects. These tokens are all in the same lexical category: symbol.
When Lisp read syntax is scanned and turned into an object, such as a nested list representing program code, this is done without regard for the semantics (what the symbols mean).
This is different from the parsing of languages (such as some of those in the broad Fortran/Algol family) which have reserved keywords.
Roughly speaking, reserved keywords are tokens which look like symbols but are actually just punctuation. Lisp has punctuation also, like parentheses, sharpsign prefixes, various quotes and such.
These punctuation words have a fixed role in the phrase structure grammar, and the phrase structure grammar must be processed before the semantics of the program can be considered.
So for instance, the reserved BEGIN and END keywords in Pascal are essentially nothing more than verbose parentheses. The '(' and ')' tokens are similarly reserved in Lisp-like languages. Trying to use BEGIN as the name of a function or variable in Pascal is similar to trying to use ( as the name of a function or variable in Lisp.
Some languages have keywords which determine phrase structure, yet allow identifiers which look exactly like reserved keywords to be used anyway. For instance, PL/I was famous for this:
IF IF=THEN THEN THEN=ELSE; ELSE ELSE=IF
Lisp dialects may assign special semantic treatment to certain symbols or certain categories of symbols. This is a sort of reservation, but not exactly the same as reserved keywords, because it is at the semantic level. For instance, in Common Lisp, the symbols nil and t (more specifically the nil and t in the common-lisp package, common-lisp:nil and common-lisp:t) may not be used as function or variable names. When either one appears as an expression, it evaluates to itself: the value of t is t and that of nil is nil. Moreover, nil is also the Boolean false value and the empty list. So, effectively, these symbols are reserved in some regards. Common Lisp also has a keyword package. All symbols in that package evaluate to themselves and may not be used as variables. They may be used as function names, and for any other purpose.
You say Lisp, but the answer changes depending on which Lisp you're talking about.
In Common Lisp, you can use list as a variable because Common Lisp is a Lisp-2, meaning that each symbol has a separate slot for a function binding and a variable binding. Common Lisp sets the function binding for the symbol list in the CL package, but doesn't set the variable binding. You can't change the function binding because Common Lisp doesn't allow you to redefine bindings for symbols that are set in the CL package (you can, of course, use whatever symbols you like in your own packages), but since the variable binding is free you're allowed to use it.
Scheme is a Lisp-1, which means that it only has one binding per symbol. There's no separation of function bindings and variable bindings (hence why you use define in Scheme, but defun and defvar in CL). The reason you can use "list" as a variable is because Scheme doesn't prevent you from rebinding its built-in symbols. It's just generally a bad idea, since by redefining list you can no longer call the list function.
Emacs Lisp is a Lisp-2 but doesn't prevent you from rebinding symbols, which means you can do things like (defun + (- a b)) and totally screw up your editing session. So... don't do that, unless you really know what you're doing.
Clojure is a Lisp-1. I don't have a working Clojure install at the moment so I can't comment on what it lets you do. I would suspect it's more strict than Scheme.
Why does Rubocop / the community-driven Ruby style guide recommend parentheses in method definitions?
def my_method(param1, param2)
end
# instead of
def my_method param1, param2
end
Method calls are allowed with or without parentheses depending on the situation. However, my first impression is that lack of parentheses in method calls are much more potentially ambiguous than lack of parentheses in method definitions. Was there a reason behind it, e.g. to make code more fool-proof, or did it happen because of "historical reasons" or "because it was the most widespread style"?
Clarification:
I am not asking for opinions about which style is easier to read.
The lint Lint/AmbiguousOperator is based on the idea that do_something *some_array is ambiguous and a source for bugs (Link). I wondered if this is the same case for Style/MethodDefParentheses (Link).
After going back to find the actual names of those Cops, my best guess right now is that there is no "technical" reason, but rather one is a proper "lint" and the other a "style" matter.
The rationale is omitted in the initial commit, of which this rule was part, indicating that there was no particular technical reason for it.
The fact that the corresponding cop is placed in the Style department, rather than Lint, serves as further proof that this is a matter of just that, style.
Method definitions have a very simple syntax. The def keyword is (optionally) followed by arguments, which must be followed by a terminator (newline or ;).
The possible variations are:
single line method definitions,
inline modifiers, e.g. private,
default- and keyword arguments,
splat- and block arguments.
All of these work fine both with and without parentheses. Furthermore, running a file with an unparenthesized method definition using the -w flag raises no warnings.
These factors together rule out the possibility that the parentheses are recommended to avoid ambiguity.
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 ;)
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.
I want to know it real meaning of it and how to use it exactly.
Another question is about assert I saw
assert product.valid? product.errors.full_messages
and
assert product.valid?
But I can't find syntax for those assert what does second arg for assert (product.errors.full_messages) means or it is arg for ?
Thanks
Ruby uses specific naming conventions for methods. It allows you to quickly identify the side effects they may have, or the return type..
These conventions use special markers such as "!" and "?" at the end of method names. This is uncommon since most programming languages tend to forbid such characters in identifiers, but nevertheless, it is truly part of the method name. (and should not be confused with operators)
Post fixing "?" means that the method returns a boolean. It's a convenient way to replace the "is" prefix. (this convention tends to exist in lisp dialects too)
Post fixing "!" means that the method will modify the instance, and thus won't act on/return a copy.
Note that these are just conventions. In no way you have to follow it, but it is considered a good practice.
It is purely a convention, it has no formal meaning. It isn't unusual for people to get confused by this, since many (most?) languages don't allow characters such as ? and ! in identifiers.
It's just convention to put it on a method that returns a boolean afaik. Rather than making everythign IsSomething.