Pohtimassa <> merkkien käyttöä, T, E , DART:issa

Lisäksi DART kielessä on näitä <T>   <E>  ilmaisuja.  Muiden sulkumerkkien jälkeen ne pysähdyttävät hieman miettimään...    Miksi juuri  T   ja  E  ?
(K,V)  Tarkoittaa siis Key ja Value,  sehän on selvää.
Kokoan leikkeitä ohjeista:

http://www.dartlang.org/language-tour/#generics

Generics

If you look at the API documentation for the basic array type, List, you'll see that the type is actuallyList<E>. The <...> notation marks List as a generic (or parameterized) type—a type that can declare formal type parameters.

For example, if you intend for a list to contain only strings, you can declare it as List<String> (read that as "List of String"). 

....

Generic types can save you the trouble of creating all these interfaces. Instead, you can create a single interface that takes a type parameter:

interface Cache<T> {
  T getByKey(String key);
  setByKey(String key, T value);
}

In this code, T is the stand-in type. It's a placeholder that you can think of as a type that a developer will define later.

Using collection literals

Both built-in collection types are parameterized: lists and maps. Parameterized literals are just like the literals you've already seen, except that you add <type> before the opening bracket. For example:

List<String> names = <String>['Seth', 'Kathy', 'Lars'];
Map<String, String> pages = <String>{ // specify value type: String
    'index.html':'Homepage',  // (the key type is implicitly String)
    'robots.txt':'Hints for web robots',
    'humans.txt':'We are people, not machines' };

NO?  Näyttää siltä siis, että <> merkinnän sisällä on "tyyppi"...

..

To specify one or more types when using a constructor, put the types in angle brackets (<...>

..

Generic collections and the types they contain

Dart generic types are reified, which is a fancy way of saying that they carry their type information around at runtime. 

= = = = = = = = = = = = = = = = = = = = = = = = = 




http://api.dartlang.org/dart_core.html

• Collection<E>    
• HashSet<E>
• Iterable<E>
• Iterator<E>
• List<E>
• Queue<E>
• Set<E>  
• 

• 

• HashMap<K, V>
• LinkedHashMap<K, V>
• Map<K, V>       mapping a key to a value.
• 

• 

• Completer<T>
• Future<T>




SIIS:  E   T   K   V 

What is this T  in <T>  ?

Interface Future<T>

A Future is used to obtain a value sometime in the future. Receivers of a Future obtain the value by passing a callback to then. 




========================================================

What is this   E   in <E>  ?  




===========================================




http://www.dartlang.org/language-tour/#Typedefs

Typedefs can be parameterized.

typedef int Compare<T>(T a, T b);

class SortedCollection<T> {
  Compare<T> compare;
  SortedCollection(this.compare);
}

main() {
  SortedCollection<int> s = new SortedCollection<int>((a,b) => a - b);
  print(s.compare is Compare<int>);  // true
  print(s.compare('a','b'));  // checked mode throws exception
}

dartlang  0.08

typeParameter:
identifier (extends type)?
;
typeParameters:
`<' typeParameter (`,' typeParameter)* `>'
;
A type parameter T may be suffixed with an extends clause that specifies
the upper bound for T.

A type parameter T may be suffixed with an extends clause that specifies
the upper bound for T.   ....

Tässäpä hieman sulattelemista...    Täydennän tekstiä vielä...  


http://c.dart-examples.com/learn/variables/collections
Tämä kirjoitus tuo hieman lisävalaistusta asiaan...

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