look at this code :
function a2j(trusted:Boolean=true):String
{
...
}
compiler will not accept this code in flash actionscript 2.
It looks like AS2 doesn't force you to supply the all the arguments that a function declares. At the bottom of this help page, they state that arguments you do not supply are undefined ... and that any extra arguments you supply are ignored.
Also, the answer to this question shows that you can use the arguments keyword (an Array) to work with the parameters that are passed into the function.
So for a default value, as in your example above, you could do something like this:
function methodThatHasADefault(value:Boolean):void
{
if (arguments.length == 0)
value = true;
// do something
}
Related
Is there a construct that communicates to the type checker a function's post-condition?
For example, in typescript it is possible to say
function assertIsNumber(value: any): asserts value is number {
if (typeof value !== 'number') {
throw new TypeError();
}
}
I would like to be able to do something like the following in dart:
class SomeClass {
int? value;
_checkPreconditions() {
if(value == null) {
throw MyPreconditionError()
}
// ...
}
somefunc() {
_checkPreconditions();
// here we know `value` is a non-null int.
final sum = value + 5;
}
}
I understand I could coerce the value to non-null sum = value! + 5, but I would prefer to allow the function to inform the type checker if possible.
It looks like the type system of Dart is not so powerful. The only thing that looks (from first glance) possible is to create a custom code analyzer package (or search for one that already exists).
Dart annotations don't actually do anything. They provide hints to tools such as the Dart analyzer (usually so that it can generate additional warnings), but they cannot change program behavior. Even if you could convince the analyzer to treat some variables as different types, you still wouldn't be able to compile and run your code.
Annotations can be used by code generation tools, so one possibility might be to generate a statement such as final value = this.value!; automatically. However, that would be a lot of trouble to go through (and would mean that code then would need to use this.value = 42; for assignments and would prevent your code from being analyzed directly).
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.
In reference to adding module in micropython, I was trying to create a class which has a local method. In the documentation it is given how to add local methods and that the first argument should be of mp_obj_t type which is the data struct itself. However, I was asking how can I pass extra parameters like other methods? I tried using mp_obj_t * args as second argument but STATIC MP_DEFINE_CONST_FUN_OBJ_1 gives error. I tried the same with STATIC MP_DEFINE_CONST_FUN_OBJ_VAR but it does not support passing mp_obt_t as first argument as STATIC MP_DEFINE_CONST_FUN_OBJ_VAR needs an int. I am quite new, so I was asking how to add methods to classes which can accept arguments?
You need MP_DEFINE_CONST_FUN_OBJ_2, since you have 2 arguments.
Something like
STATIC mp_obj_t my_class_func(mp_obj_t self, mp_obj_t arg) {
if (MP_OBJ_IS_SMALL_INT(lhs)) {
const mp_int_t lhs_val = MP_OBJ_SMALL_INT_VALUE(arg);
//...
} else {
//oops, not an int
}
return mp_const_none;
}
MP_DEFINE_CONST_FUN_OBJ_2(my_class_func_obj, my_class_func);
The best source of samples like this is the source code btw.
To eleaborate on #stijn answer ~ when creating a class, all the MP_DEFINE_CONST_FUN_OBJ_XXXXXX defines work the exact same as they would if you weren't creating a class. The only difference is the first argument of ACTUAL arguments will always refer to self
Here's an example:
mp_obj_t Class_method(mp_uint_t n_args, const mp_obj_t *args) { ... }
That is the standard candidate for:
MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(Class_method_obj, 1, 3, Class_method);
However, in this case args[0] will be self.
Let's have another example.
mp_obj_t Class_method(mp_uint_t n_args, const mp_obj_t *args, mp_map_t *kw_args) { ... }
That's a prime candidate for this define
MP_DEFINE_CONST_FUN_OBJ_KW(Class_method_obj, 2, Class_method);
The only difference in this case is that the first index of allowed_args needs to automatically be handled as self. Nothing about how you do these things changes, except now the first ACTUAL argument (ie not including n_args or any other "helper" argument) needs to automatically be considered as self. That being said, you will NEVER use MP_DEFINE_CONST_FUN_OBJ_0 with a class method. '_0' means "zero arguments" and a class method will never have zero arguments because it will ALWAYS at least have self. This also means that you have to add one to however many expected arguments you have on the python end. If your python version accepts 3 arguments ~
(red, green, blue)
then your C_MODULE define has to start at 4 because it's going to get
(self, red, green, blue)
I can't seem to find a concise answer to this question. What is the correct coffeescriptic way to return the value from _otherInstanceMethod when calling #_instanceMethod instead of the function binding itself?
x = _instanceMethod: () ->
#_otherInstanceMethod key: 'value'
Edit (thanks commenters)
This returns:
x = function () {
[...] # function body omitted
});
Instead of
x = 'some value returned by _otherInstanceMethod'
I would like the value to be returned instead of the function binding to _otherInstanceMethod
Being totally new to Coffeescript, this was my fault. I was calling the instance method like:
#_instanceMethod
instead of
#_instanceMethod()
Sorry for the trouble, voting to delete
In CoffeeScript #something translated into this.something regardless of the underlying variable type. This means you can use # only in conjuction with properties, with methods you still ought to use good old this.
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
}