All I need to do with my code is if it passes the if statement, it simply passes the class and moves on to the rest of the code, but in my research, I can't exactly figure out what the command may be. Is there a way to implement Python's pass keyword in dart?
There is no direct equivalent to the pass statement in Dart language. In Python, there are language restrictions that do not allow leaving empty loops, if statements and so on. However, in Dart it's completely OK to do that, you can just leave empty braces. For instance:
void main() {
for (int i = 0; i < 5; i++) {
// Ignored
}
try {
// Some logic
} on Exception {
// Ignored
}
if (true) {
// Ignored
}
}
There are even some lint rules that check for you not to leave empty logic (you can still add a comment and not leave them empty, though): empty_statements, empty_catches and so on.
Just add empty curly braces {}. For example:
const num = 5;
if (num == 5) {
}
you don't have to provide code within the curly braces, it can be left empty.
Related
I know that in the latest version of dart we can use if else statements inside the build method. Does anyone know if we can use also if else statement when we setting class parameters? I know I can do inline statement there but inline is a bit hard to read when there are multiple conditions
const int i = 0;
class Person {
// NewClass n = NewClass(a: i == 0 ? 'a' : 'b'); //<- inline statement working
NewClass n = NewClass(a: if(i == 0) 'a' else 'b'); //<- if statement doesn't
}
class NewClass {
final String a;
const NewClass({this.a});
}
Edit:
Basically in my case I've got an TextField widget where I set its's type parameter from enum (Type.text, Type.numeric...) According to this parameter I want to set The textField parameters (textCapitalization, maxLength and so on)
As per your comment, you are already creating an enum for specifying the type of the fields.
enum Type {text, numeric}
Now for specifying the properties of that particular type, you can add an extension on this enum, as shown below:
extension TextFieldProperties on Type {
int get maxLength {
if (this == Type.text) {
return 10;
}
return 12;
}
}
So in your field class you already have a type defined, you can use that type variable to get the properties of that particular type of field.
Type type = Type.text;
print(type.maxLength); // Will print 10
type = Type.numeric;
print(type.maxLength); // Will print 12
Note: It will work only in Dart 2.7 and above
You want the conditional expression (?:), not the conditional statement or literal entry (if), as you have already discovered.
The reason if doesn't work is that if only works as a statement or as a collection literal entry. It doesn't work in arbitrary expressions.
The reason for the distinction is that the if syntax allows you to omit the else branch. That only makes sense in places where "nothing" is a valid alternative. For a statement, "doing nothing" is fine. For a collection, "adding nothing" is also fine.
In an expression context, you must evaluate to a value or throw. There is no reasonable default that we can use instead of "nothing", so an if is not allowed instead of an expression.
Doesn't work because this syntax doesn't exist in Dart. The only way to do what you would like to do is to use the ternary operator.
If you try it in the DartPad you will get an error.
I suggest you to use a function to return the right value.
Is it possible to write a macro that expands an expression into multiple indexed arguments, which can be passed to a function or another macro?
See this simple self contained example.The aim is to have unpack3 expand v into v[0], v[1], v[2].
macro_rules! elem {
($val:expr, $($var:expr), *) => {
$($val == $var) || *
}
}
// attempt to expand an array.
macro_rules! unpack3 {
($v:expr) => {
$v[0], $v[1], $v[2]
}
}
fn main() {
let a = 2;
let vars = [0, 1, 3];
// works!
if elem!(a, vars[0], vars[1], vars[2]) {
println!("Found!");
}
// fails!
if elem!(a, unpack3!(vars)) {
println!("Found!");
}
}
The second example fails, is it possible to make this work?
Possible solutions could include:
Changing use of macro grammar.
Using tuples, then expanding into arguments after.
Re-arranging the expressions to workaround macro constraints.
Note, this may be related to Escaping commas in macro output but don't think its a duplicate.
This is impossible in two different ways.
First, to quote the answer to the question you yourself linked: "No; the result of a macro must be a complete grammar construct like an expression or an item. You absolutely cannot have random bits of syntax like a comma or a closing brace." Just because it isn't exactly a comma doesn't change matters: a collection of function arguments are not a complete grammar construct.
Secondly, macros cannot parse the output of other macros. This requires eager expansion, which Rust doesn't have. You can only do this using recursion.
I am looking over an Xcode project I downloaded and am seeing code syntax that I am unfamiliar with:
The braces don't belong to a method signature, or any other conditional statement, they are just floating there. What is the point of this? Purely for code segregation/readability purposes?
This is just block scope; and is the same in C and C++. Any variables declared within the block are inaccessible outside of it. I commonly use it in switch statements:
switch(x) {
case 1: {
const char *s = "hi";
}
break;
case 2: {
const char *s = "ho";
}
break;
// etc.
}
Note that there are two variables called s, neither of which interfere with the other as they are within their own scope.
The declarations within the scope enclosed by the braces will be confined to that scope, so label, icon, and button will not be visible outside of it. As such it provides locality which is generally considered to be good.
Legacy code needed { } in order to do declarations at all
In C89, you couldn't just do int i; anywhere; declarations were only valid at the beginning of blocks.
check this for more explanation
Why enclose blocks of C code in curly braces?
I'm trying to translate some old code to Coffeescript. But there is no direct translation for:
while ( doWork() ) {}
"while doWork()" with nothing after it results in a syntax error.
while doWork() then
Should do the trick
using then is probably the canonical solution since it is explicitly meant for separating the condition from the (in this case empty) body. Alternatively you can write
while doWork()
;#
(the # keeps vim syntax highlighting from flagging it as an error)
I also like the continue while doWork() solution, but I strongly advise against any other form of expression while doWork() mentioned in the comments since when this is the last statement of a function it will become a list constructor:
_results = [];
while (doWork()) {
_results.push(expression);
}
return _results;
I'm trying to write a class for a scala project and I get this error in multiple places with keywords such as class, def, while.
It happens in places like this:
var continue = true
while (continue) {
[..]
}
And I'm sure the error is not there since when I isolate that code in another class it doesn't give me any error.
Could you please give me a rule of thumb for such errors? Where should I find them? are there some common syntactic errors elsewhere when this happens?
It sounds like you're using reserved keywords as variable names. "Continue", for instance, is a Java keyword.
You probably don't have parentheses or braces matched somewhere, and the compiler can't tell until it hits a structure that looks like the one you showed.
The other possibility is that Scala sometimes has trouble distinguishing between the end of a statement with a new one on the next line, and a multi-line statement. In that case, just drop the ; at the end of the first line and see if the compiler's happy. (This doesn't seem like it fits your case, as Scala should be able to tell that nothing should come after true, and that you're done assigning a variable.)
Can you let us know what this code is inside? Scala expects "expressions" i.e. things that resolve to a particular value/type. In the case of "var continue = true", this does not evaluate to a value, so it cannot be at the end of an expression (i.e. inside an if-expression or match-expression or function block).
i.e.
def foo() = {
var continue = true
while (continue) {
[..]
}
}
This is a problem, as the function block is an expression and needs to have an (ignored?) return value, i.e.
def foo() = {
var continue = true
while (continue) {
[..]
}
()
}
() => a value representing the "Unit" type.
I get this error when I forget to put an = sign after a function definition:
def function(val: String):Boolean {
// Some stuff
}