I see multiple forms of typedef throughout dart and flutter libraries, but I can't quite make sense of it. There's this example in framework.dart:
typedef ElementVisitor = void Function(Element element);
And there's this example (https://medium.com/#castellano.mariano/typedef-in-dart-40e96d3941f9):
typedef String Join(String a, String b);
I don't quite understand the difference of their uses. Maybe this has something to do with why I can't find the definition of "Function" anywhere in the Dart or Flutter libraries. But then again I can find other typedef's just fine in the framework.dart file.
As docs refers
There is an old and new way of typedef
in general: the new way is a bit clearer and more readable.
in details:
typedef G = List<int> Function(int); // New form.
typedef List<int> H(int i); // Old form.
Note that the name of the parameter is required in the old form, but the type may be omitted. In contrast, the type is required in the new form, but the name may be omitted.
As well as
The reason for having two ways to express the same thing is that the new form seamlessly covers non-generic functions as well as generic ones, and developers might prefer to use the new form everywhere, for improved readability.
There is a difference between declaring a generic function type and declaring a typedef which takes a type argument. The former is a declaration of a single type which describes a certain class of runtime entities: Functions that are capable of accepting some type arguments as well as some value arguments, both at runtime. The latter is a compile-time mapping from types to types: It accepts a type argument at compile time and returns a type, which may be used, say, as a type annotation. We use the phrase parameterized typedef to refer to the latter. Dart has had support for parameterized typedefs for a while, and the new syntax supports parameterized typedefs as well. Here is an example of a parameterized typedef, and a usage thereof:
typedef I<T> = List<T> Function(T); // New form.
typedef List<T> J<T>(T t); // Old form.
I<int> myFunction(J<int> f) => f;
For more info
A typedef can be used to specify a function signature that we want specific functions to match. A function signature is defined by a function’s parameters (including their types). The return type is not a part of the function signature.
typedef function_name(parameters)
A variable of typedef can point to any function having the same signature as typedef.
typedef var_name = function_name
One declared a type while other assigns it to a typedef variable
https://dart.dev/guides/language/language-tour#typedefs
Okay I know "One Definition Rule", but when I try to declare a variable with different types subsequently in source code, I run into some mistake like following:
int fkc();
void fkc();
enter image description here
I mean these two statements are just two declarations, not definitions. Alright, Does every declaration have to have only one unique definition?
In C++, you can't overload functions based on return type.
Overload resolution takes into account the function name , cv-qualifiers , the number of parameters and their types.
You could do something like:
auto fck()
{
if constexpr(...) return my_int;
else /* do smth without return */
}
but that's not function overloading of course.
Because you cannot overload the method just by changing return type . It is not allowed. The compiler distinguishes function invocations based on signature.and the signature of function includes only function name and arugments like
func(int x....) which does not include return type
I have a function(myFunction) with parameter v, I hope it can return different object depend the value of v.
something like below:
-(NSString*)myFunction:(NSInteger)v;
-(NSNumber*)myFunction:(NSInteger)v;
Is it possible?
Welcome any comment
Thanks interdev
You can always return an id, but I think it could be confusing for whoever is using this function, as they wouldn't know what the function would return.
Depending on the context you're using the function, it would probably be better to simply have different functions
-(NSString*)myFunctionNSStringForNSInteger:(NSInteger)v;
-(NSNumber*)myFunctionNSNumberForNSInteger:(NSInteger)v;
If you want it to do different things depending on the value of v why not just switch on the value of v and call the relevant function with the required return type?
You can e.g. declare the function to return NSObject. Then you can return all kinds of objects that inherits from NSObject. When using the function you might have to check the type of the returned object, depending on what you want to do with the result.
I'm a bit confused about the use of all the IEnumerable<T> extension methods, intellisense is always asking for <T>, but I don't think it's necessary to specify <T> at all times.
Let's say I have the following:
List<Person> people = GetSomePeople();
How is this:
List<string> names = people.ConvertAll<string>(p=>p.Name).Distinct<string>().ToList<string>();
different from this:
List<string> names = people.ConvertAll<string>(p=>p.Name).Distinct().ToList();
I think the two lines of code above are sxactly the same, now the question:
How do I know when to specify <T> and when to skip it?
The simplest way is obviously to omit it and see if it compiles.
In practice, you can omit type parameters wherever they are inferred; and they can normally be inferred when they are used in the type of a method parameter than you specify. They cannot be inferred if they're used only in the return type of the method. Thus, for example, for Enumerable.Select<T>, T will be inferred from the type of first argument (which is of type IEnumerable<T>). But for Enumerable.Empty<T>(), will not be inferred, because it's only used in return type of the method, and not in any arguments (as there are none).
Note that the actual rules are more complex than that, and not all arguments are inferable. Say you have this method:
void Foo<T>(Func<T, T> x);
and you try to call it with a lambda:
Foo(x => x);
Even though T is used in type of argument here, there's no way to infer the type - since there are no type specifications in the lambda either! As far as compiler is concerned, T is the same type x is, and x is of type T...
On the other hand, this will work:
Foo((int x) => x);
since now there is sufficient type information to infer everything. Or you could do it the other way:
Foo<int>(x => x);
The specific step-by-step rules for inference are in fact fairly complicated, and you'd be best off reading the primary source here - which is C# language specification.
This feature is known as type inference. In your example, the compiler can automatically determine the generic argument type implicitly for you because in the method call to ConvertAll, the parameter lambda returns a string value (i.e. Name). So you can even remove the <string> part of ConvertAll call. The same is with Distict(), as ConvertAll returns a List<string> and the compiler can declare the generic argument for you.
As for you answer, when the compiler can determine the type itself, the generic argument is redundant and unnecessary. Most of the times, the only place where you need to pass the generic argument is the declaration, like, List<string> list = new List<string>();. You can substitute the first List<string> with var instead or when you are using templates as parameters in lambdas too.
Can I specify interfaces when I declare a member?
After thinking about this question for a while, it occurred to me that a static-duck-typed language might actually work. Why can't predefined classes be bound to an interface at compile time? Example:
public interface IMyInterface
{
public void MyMethod();
}
public class MyClass //Does not explicitly implement IMyInterface
{
public void MyMethod() //But contains a compatible method definition
{
Console.WriteLine("Hello, world!");
}
}
...
public void CallMyMethod(IMyInterface m)
{
m.MyMethod();
}
...
MyClass obj = new MyClass();
CallMyMethod(obj); // Automatically recognize that MyClass "fits"
// MyInterface, and force a type-cast.
Do you know of any languages that support such a feature? Would it be helpful in Java or C#? Is it fundamentally flawed in some way? I understand you could subclass MyClass and implement the interface or use the Adapter design pattern to accomplish the same thing, but those approaches just seem like unnecessary boilerplate code.
A brand new answer to this question, Go has exactly this feature. I think it's really cool & clever (though I'll be interested to see how it plays out in real life) and kudos on thinking of it.
As documented in the official documentation (as part of the Tour of Go, with example code):
Interfaces are implemented implicitly
A type implements an interface by implementing its methods. There is
no explicit declaration of intent, no "implements" keyword.
Implicit interfaces decouple the definition of an interface from its
implementation, which could then appear in any package without
prearrangement.
How about using templates in C++?
class IMyInterface // Inheritance from this is optional
{
public:
virtual void MyMethod() = 0;
}
class MyClass // Does not explicitly implement IMyInterface
{
public:
void MyMethod() // But contains a compatible method definition
{
std::cout << "Hello, world!" "\n";
}
}
template<typename MyInterface>
void CallMyMethod(MyInterface& m)
{
m.MyMethod(); // instantiation succeeds iff MyInterface has MyMethod
}
MyClass obj;
CallMyMethod(obj); // Automatically generate code with MyClass as
// MyInterface
I haven't actually compiled this code, but I believe it's workable and a pretty trivial C++-ization of the original proposed (but nonworking) code.
Statically-typed languages, by definition, check types at compile time, not run time. One of the obvious problems with the system described above is that the compiler is going to check types when the program is compiled, not at run time.
Now, you could build more intelligence into the compiler so it could derive types, rather than having the programmer explicitly declare types; the compiler might be able to see that MyClass implements a MyMethod() method, and handle this case accordingly, without the need to explicitly declare interfaces (as you suggest). Such a compiler could utilize type inference, such as Hindley-Milner.
Of course, some statically typed languages like Haskell already do something similar to what you suggest; the Haskell compiler is able to infer types (most of the time) without the need to explicitly declare them. But obviously, Java/C# don't have this ability.
I don't see the point. Why not be explicit that the class implements the interface and have done with it? Implementing the interface is what tells other programmers that this class is supposed to behave in the way that interface defines. Simply having the same name and signature on a method conveys no guarantees that the intent of the designer was to perform similar actions with the method. That may be, but why leave it up for interpretation (and misuse)?
The reason you can "get away" with this successfully in dynamic languages has more to do with TDD than with the language itself. In my opinion, if the language offers the facility to give these sorts of guidance to others who use/view the code, you should use it. It actually improves clarity and is worth the few extra characters. In the case where you don't have access to do this, then an Adapter serves the same purpose of explicitly declaring how the interface relates to the other class.
F# supports static duck typing, though with a catch: you have to use member constraints. Details are available in this blog entry.
Example from the cited blog:
let inline speak (a: ^a) =
let x = (^a : (member speak: unit -> string) (a))
printfn "It said: %s" x
let y = (^a : (member talk: unit -> string) (a))
printfn "Then it said %s" y
type duck() =
member x.speak() = "quack"
member x.talk() = "quackity quack"
type dog() =
member x.speak() = "woof"
member x.talk() = "arrrr"
let x = new duck()
let y = new dog()
speak x
speak y
TypeScript!
Well, ok... So it's a javascript superset and maybe does not constitute a "language", but this kind of static duck-typing is vital in TypeScript.
Most of the languages in the ML family support structural types with inference and constrained type schemes, which is the geeky language-designer terminology that seems most likely what you mean by the phrase "static duck-typing" in the original question.
The more popular languages in this family that spring to mind include: Haskell, Objective Caml, F# and Scala. The one that most closely matches your example, of course, would be Objective Caml. Here's a translation of your example:
open Printf
class type iMyInterface = object
method myMethod: unit
end
class myClass = object
method myMethod = printf "Hello, world!"
end
let callMyMethod: #iMyInterface -> unit = fun m -> m#myMethod
let myClass = new myClass
callMyMethod myClass
Note: some of the names you used have to be changed to comply with OCaml's notion of identifier case semantics, but otherwise, this is a pretty straightforward translation.
Also, worth noting, neither the type annotation in the callMyMethod function nor the definition of the iMyInterface class type is strictly necessary. Objective Caml can infer everything in your example without any type declarations at all.
Crystal is a statically duck-typed language. Example:
def add(x, y)
x + y
end
add(true, false)
The call to add causes this compilation error:
Error in foo.cr:6: instantiating 'add(Bool, Bool)'
add(true, false)
^~~
in foo.cr:2: undefined method '+' for Bool
x + y
^
A pre-release design for Visual Basic 9 had support for static duck typing using dynamic interfaces but they cut the feature* in order to ship on time.
Boo definitely is a static duck-typed language: http://boo.codehaus.org/Duck+Typing
An excerpt:
Boo is a statically typed language,
like Java or C#. This means your boo
applications will run about as fast as
those coded in other statically typed
languages for .NET or Mono. But using
a statically typed language sometimes
constrains you to an inflexible and
verbose coding style, with the
sometimes necessary type declarations
(like "x as int", but this is not
often necessary due to boo's Type
Inference) and sometimes necessary
type casts (see Casting Types). Boo's
support for Type Inference and
eventually generics help here, but...
Sometimes it is appropriate to give up
the safety net provided by static
typing. Maybe you just want to explore
an API without worrying too much about
method signatures or maybe you're
creating code that talks to external
components such as COM objects. Either
way the choice should be yours not
mine.
Along with the normal types like
object, int, string...boo has a
special type called "duck". The term
is inspired by the ruby programming
language's duck typing feature ("If it
walks like a duck and quacks like a
duck, it must be a duck").
New versions of C++ move in the direction of static duck typing. You can some day (today?) write something like this:
auto plus(auto x, auto y){
return x+y;
}
and it would fail to compile if there's no matching function call for x+y.
As for your criticism:
A new "CallMyMethod" is created for each different type you pass to it, so it's not really type inference.
But it IS type inference (you can say foo(bar) where foo is a templated function), and has the same effect, except it's more time-efficient and takes more space in the compiled code.
Otherwise, you would have to look up the method during runtime. You'd have to find a name, then check that the name has a method with the right parameters.
Or you would have to store all that information about matching interfaces, and look into every class that matches an interface, then automatically add that interface.
In either case, that allows you to implicitly and accidentally break the class hierarchy, which is bad for a new feature because it goes against the habits of what programmers of C#/Java are used to. With C++ templates, you already know you're in a minefield (and they're also adding features ("concepts") to allow restrictions on template parameters).
Structural types in Scala does something like this.
See Statically Checked “Duck Typing” in Scala
D (http://dlang.org) is a statically compiled language and provides duck-typing via wrap() and unwrap() (http://dlang.org/phobos-prerelease/std_typecons.html#.unwrap).
Sounds like Mixins or Traits:
http://en.wikipedia.org/wiki/Mixin
http://www.iam.unibe.ch/~scg/Archive/Papers/Scha03aTraits.pdf
In the latest version of my programming language Heron it supports something similar through a structural-subtyping coercion operator called as. So instead of:
MyClass obj = new MyClass();
CallMyMethod(obj);
You would write:
MyClass obj = new MyClass();
CallMyMethod(obj as IMyInterface);
Just like in your example, in this case MyClass does not have to explicitly implement IMyInterface, but if it did the cast could happen implicitly and the as operator could be omitted.
I wrote a bit more about the technique which I call explicit structural sub-typing in this article.