Ever been stumped by <E>, <T>, <K, V> in OO language documentation?

Over on the Reddit /r/dartlang group, an individual by the name of NFC_TagsForDroid reached out to me in regards to confusion navigating the Dart documentation. This was specifically when understanding the meaning behind some of the "tokens" used when demonstrating code samples.

Here's an extract from the user's comment:

Can you please write a explainer on how to read dartlang documentation? Most of it is meaningless to a beginner. For example: https://api.dartlang.org/stable/2.1.0/dart-core/List/add.html (Dart dart:core List<E> add abstract method)
the heading List<E> add abstract method what is the <E>? how does it being an abstract method affect anything?

The user is referring to the signature for the List type's add() method:

void add(
  E value
);

The source of confusion is the E. Others used in various documentation code samples are T, K and V.

So what do these mean?

These seemingly magical "tagged letters" are placeholders that are used to represent what is known as a type parameter. This is common across statically-typed object-oriented languages and will be apparent when it comes to the topic of Generics.

Generics provide a way of telling the compiler what type is being used, so that it knows to check for that.

In other words if you see <E> read it as "of Type", so List<String> will be read as "List of String".

Now looking at that let's say we define a List<E>:

List<String> fruits = ['apple', 'orange', 'pineapple'];

Looking at the add method again:

void add(
  E value
);

The way to read this is that the E represents an element in the collection, whatever type we initially specified when we created that list! In the case of fruits its String.

And this is how we would use it:

fruits.add('mango');

fruits.add(1); // This will throw an error as it's the wrong type

So why are particular letters used?

Simplest answer is convention. In fact you can use any letters you like, achieving the same effect. However the common ones carry semantic meaning:

• T is meant to be a Type
• E is meant to be an Element (List<E>: a list of Elements)
• K is Key (in a Map<K, V>)
• V is Value (as a return value or mapped value)

This code below will work even when I don't use any of the placeholder letters above. For example, see this snippet below:

class CacheItem<A> { // Using the letter A
  CacheItem(A this.itemToCache);

  final itemToCache;

  String get detail => '''
    Cached item: $itemToCache
    The item type is $A
  ''';
}

void main() {
  var cached = CacheItem<int>(1);
  print(cached.detail);
  // Output:
  // Cached item: 1
  // The item's type is int
}

→ Run in DartPad

This works although the placeholder used is A. By convention T would be used:

class CacheItem<T> {
  CacheItem(T this.itemToCache);

  final itemToCache;

  String get detail => '''
    Cached item: $itemToCache
    The item type is $T
  ''';
}

Generics are powerful in that they allow us to reuse the same class with various types:

CacheItem<String> cachedString = CacheItem<String>('Hello, World!');
print(cachedString.detail);
// Output:
// Cached item: Hello, World!
// The item's type is String

var cachedInt = CacheItem<int>(30);
print(cachedInt.detail);
// Output:
// Cached item: 30
// The item's type is int

var cachedBool = CacheItem<bool>(true);
print(cachedBool.detail);
// Output:
// Cached item: true
// The item's type is bool

I hope this was insightful and you learnt something new today.

Further reading

• StackOverflow post about Generics
• Language guide to generics
• Free Dart Screencasts on Egghead.io

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Comments

[edA‑qa mort‑ora‑y]

Nice practical intro.

Just to add some terms, in case people wish to look further. The overall concept here are "parametric types" and "parametric functions", where "parameter" is referring to a type, as opposed to an "argument" which refers to a value to a function.

The term "generics" generally refers to parametric container types, and some limited functions. It's a useful, but perhaps less-than-complete form of parametric types, as the implementations usually have many limitations. Full parametric allow a kind of meta-programming, C++ style. Whether this is better or not depends greatly on the experience of the programmer.

Dynamically typed languages don't need parametric types as any container or function, can take any type at all. Though there's a related concept known as duck-typing that applies.

[Jermaine]

I like this fix. Merged.

[emptyother]

Thanks! I've been using <T, U, V, ...> when i wrote generic types. Didn't put a second thought into which letters should be used (T for Type, U because T was taken, etc).

Any reason why we don't use a multi-letter word, example: Record<KEY,VALUE> or List<ELEMENT>?

[Jermaine]

I don't have a solid reason as to why multi-letter words are not used, apart from the fact that its more characters and therefore not conventional. Consequently, this could be mistaken for actual rather than placeholder types.

[Gareth Bradley]

In C# you see multi element words, like TElement or TEntity for example.

[Jermaine]

Yeah you're right. I had seen that in the docs. It's still got the placeholder letter as prefix:

public interface ISessionChannel<TSession>
{
    TSession Session { get; }
}

docs.microsoft.com/en-us/dotnet/cs...

[Amin]

Great article, never really had the courage to deep dive into Dart but I'll do one day for sure! One thing that bothers me when writing things like

class Cache<T> {}

and then reading later, or in a comment

T meaning Type

Is that often when you say meaning, it really means that you could have just written it full length and not say that (I'm not talking about you personnally, just this kind of convention in general). Why not just write directly

class Cache<CacheType> {
  public addItem(item: CacheType): void { /* ... */ }
  public getItems(): Array<CacheType> { /* ... */ }
}

So that it gets clearer for newcomers or people getting into this code someday. I like to think that my code is a form of art that another person will have to continue after me.

[Jermaine]

Hey Amin,

You can use multi-letter placeholders if you want, like the Microsoft docs, although they still prefixed the placeholder letters docs.microsoft.com/en-us/dotnet/cs...

Personally I would be careful doing CacheType as that feels like there is a class CacheType somewhere, especially looking at addItem(item: CacheType)

[Amin]

I see, hence the prefix like TCache. I never really understood that until now. Thanks Jermaine!

[Jermaine]

You're welcome Amin.

[Sandor Dargo]

Thanks for the article. This reminds me of a code review I was part of.

It was C++ code, involving some templates. Something like this:

template <
    class C, // Something meaningful
    class R, // Something else meaningful
    class D, // Something explaning D well
    class H  // And again
>
class MyClass {
// ... quite some quantity of code...
};

I asked the question if they thought about removing the comments and use meaningful names instead. The answer was: "yes we thought about it, but decided not to".

Well...

Should code read like well-written prose?

[Vince Ramces Oliveros]

Remember Dart 2.1 has int-to-double conversion? I tried this with the Tween where T is dynamic, so its either 1.0 or 0.0 or a widget, and gave me an error to the flutter engine.

For some cases, I still have doubts to use Generics.

[Evaldas Buinauskas]

V is there already. Also, read article again! It clearly says this 😉

Simplest answer is convention. In fact you can use any letters you like, achieving the same effect.

[David J Eddy]

Great intro Jermaine!

[Jermaine]

Thanks David!

[flutterCode]

Good article.thx

[Ronaldo Geldres]

Thanks for the article :D.

I have a question if you don't mind, What should I do when I have two types? Should I use T1 and T2?

[Jermaine]

You would commonly do <T, U>, but really any letters can be used. I like the .NET constraints detailed here.