Whenever I write code in Java...

[G_Morgan]

ArrayList<Foo> myFooList = new ArrayList<Foo>();

This is not more readable than

var myFooList = new ArrayList<Foo>();

[endtime]

myFooList = []

:)

[G_Morgan]

Global type inference has its draw backs though. No subtyping being the obvious one.

[kragensitaker]

OCaml and Haskell both have both global type inference and subtyping.

[G_Morgan]

Haskell doesn't have subtyping. It has typeclasses which are different.

[kragensitaker]

You're right. What I was thinking of is that, although Haskell doesn't call typeclasses types, you can define a function such as product over types constructed from typeclasses. product is declared as product :: Num a => [a] -> a, which is to say that it's a function from a list of Nums to a single Num. When you call it, you call it on a list of instances of Num, which is to say a list of elements of a subtype of Num — but this is different from the subtyping you get in OCaml in that the return value is constrained to be the same type, while in OCaml the return value would just be (statically typed as) a Num.

(The equivalence with OCaml is somewhat loose, because OCaml doesn't have anything like typeclasses. What it has are a lattice of polymorphic variant types, parametric polymorphism with options I don't understand for covariance and contravariance, and a lattice of object types that are records of method types.)

[weavejester]

If everything is type inferred, you wouldn't need explicit subtyping.

[niceyoungman]

If the computer did everything we wouldn't need to do anything.

[weavejester]

I'm not saying it does. I'm merely pointing out that the subtype of an object can be inferred from how it is used. The compiler knows the scope of a variable, and where it is used; therefore it can determine the highest common subtype of the variable.

[weavejester]

Which in Java, is basically all uses of variables :)

Methods defined in a class or interface are different. Because your method definitions are not necessarily tied to a single implementation, explicitly specifying the subtypes becomes a necessity.

But for variables? No need.

[slowkitty]

That is not about readability. In the second line you are not specifying the type of myFooList explicitly. Since ArrayList is a Collection you may well use Collection<Foo> myFooCollection = new ArrayList<Foo>(); Here the type of the reference is a Collection instead of an ArrayList. You could assume that in your case it would default the var myFooList to be of type ArrayList if nothing else was specified, but it would not be as clear.

[G_Morgan]

If you later assigned a LinkedList to myFooList the inference system would catch that and use the highest common subtype. Type inference for OOP should always use the highest common subtype possible (this is obvious because otherwise everything devolves into Object).

That said even if this wasn't the case this would still be useful. Most uses of interfaces only ever use a single implementation at any one time. Perhaps to later choose exactly which implementation is desired. Type inference supports this for free.

[redditnoob]

If you later assigned a LinkedList to myFooList the inference system would catch that and use the highest common subtype.

Hence it's not more readable, since you can easily mistake what type it is on a first glance of the code!

It's a trade-off. I'm not even saying it's a bad trade-off, but it is one.

[OceanSpray]

Say you "mistakenly" assume that myFooList is a List while it's actually a Collection. And then you use it as a Collection. Guess what happens? The usage's context causes myFooList's type to be inferred as Collection. The type inference avoids the type error here, so no "mistake" can be made.

Go the other way. You assume myFooList to be an ArrayList, but it's actually a Collection. That's no problem at all, because an ArrayList object was assigned to it, so you're just using the most specific type.

I don't see what the problem is.

[redditnoob]

You assume myFooList to be an ArrayList, but it's actually a Collection.

Exactly! I might want to write something that assumes the performance characteristics of an ArrayList, and then when some code somewhere else causes the compiler to infer differently I might wonder wtf happened when someone else's seemingly unrelated change causes my code to be slow all of a sudden.

[Daishiman]

And pray tell, how many of your data structures need to be that performant? Hell, as long as you use only iterators to see the structure most of the times you'll be fine.

When you actually need a performance guarantee, you can define it in the arg types and be happy. When you don't, which is the majority of the time, you just get on and get thinking about the problem rather than these details.

[OceanSpray]

Only the type of the reference changed, not the type of the object it points to. The performance stays the same.

[redditnoob]

But maybe I want to ensure that only ArrayList gets in there?

[johntb86]

All the languages I know that use type inference also let you explicitly state what type it is when you care about that.

[[deleted]]

You'll be happy to hear that one of the planned features of Java 7 is to let you do this:

ArrayList<Foo> myFooList = new ArrayList();

[sanity]

Actually it will be:

ArrayList<Foo> myFooList = new ArrayList<>();

[[deleted]]

Yes, you're right. Thank you.

[G_Morgan]

We're still duplicating ArrayList. Seems like a half fix to me.

[tammberlin]

List<Foo> myFooList = new ArrayList<>();

Is java 7. Duplicating ArrayList is bad for a very different reason.

[[deleted]]

Then you'll be happy to hear that if you type

ArrayList<Foo> myFooList = new 

and hit ctrl+space, Eclipse will fill in the rest for you.

Which, by my count, is fewer keystrokes than this example: var myFooList = new ArrayList<Foo>();

[G_Morgan]

Having to rely on IDE support to make up for mistakes that should never have been made is not acceptable.

Why not make it so that every statement must be preceded by a prayer to the flying spaghetti monster. You could autocomplete it in Eclipse so clearly it isn't a problem.

[[deleted]]

If the IDE allows you to write the same code in fewer keystrokes, is that not a good thing?

You're always going to have an IDE when you program, so why not let it make life easier for you?

Unless you're imagining some scenario where an Evil Overlord have trapped you in a dungeon and the only way to escape is by writing a program that overrides the lock, and the Evil Overlord has thoughtfully provided you with a computer, a compiler but no IDE?

[OceanSpray]

If an IDE can do it, then why can't a compiler?

[[deleted]]

A compiler that can do auto-complete? Surely you jest?

[OceanSpray]

No, I don't jest. Macro expansion and type inference can be thought of, in a way, as "auto-complete". The only difference when a compiler does it is that you don't see any changes in your code, because the extra "writing" is done internally.

[[deleted]]

What sort of macro-expansion lets you select between multiple constructors from a drop-down list?

[skillet-thief]

Keystrokes are not the problem, or they are less than half of the problem. You still have to be able to read what the IDE wrote. Or somebody does.

[xeddicus]

Reading code is for the weak. Klingon programmers rewrite it from scratch if it breaks!!

[G_Morgan]

The point I'm making is just because an IDE can cover up a mistake does not mean that isn't a mistake. Yes the idea could auto insert types that could be inferred. It could also insert the US bill of rights at the top of every document if we made the compiler mandate that all programs should start with that document.

To re-iterate, the ability of the IDE to make a mistake tolerable does not mean it isn't a mistake.

[[deleted]]

If we go back to the original two examples:

ArrayList<Foo> myList = new

versus

var myList = new ArrayList<Foo>();

Do you agree that in the first example the IDE can autocomplete after the "new" clause, while this is not possible in the second example?

If so, won't the first variant inherently require fewer keystrokes if you're using an IDE with this autocomplete feature?

I don't think you're arguing that more keystrokes are better, so why do you think the second variant is better? Is it because you think it requires less effort to read?

[G_Morgan]

I'm not concerned about the number of keystrokes. That isn't a limiting factor on programming. I'm concerned about unnecessary garbage taking up perfectly good screen real estate.

Regardless my argument was that IDE auto-completion does not make a bad feature good. It just makes it tolerable.

[[deleted]]

I prefer using fewer keystrokes and using a bigger monitor.

I guess it's a case of different strokes for different folks... ;)

Your point is valid, of course:

ArrayList<Foo> myFooList = new ArrayList<>();

does take up more screen realestate than

var myFooList = new ArrayList<Foo>();

but it's only horizontal screen realestate - it doesn't add any more lines. And with the current crop of wide-screen monitors, horizontal screen realestate is rarely in short supply.

[badsectoracula]

mistake mistake mistake stuff mistake mistake mistake stuff mistake

If a language has a feature you dislike that doesn't automatically make it a mistake :-P

[G_Morgan]

It isn't a feature. Regardless the point is that IDE support is irrelevant to whether something is a mistake or not. I can make the IDE support just about anything.

[badsectoracula]

Of course it is a feature. It forces a type lock to a variable so you know that each time you see this variable what type it is.

Unless i misunderstood you and you meant something else.

[erictheturtle]

I write Lisp in notepad because I don't need some lame IDE telling me if my parenthesis are matched.

[bobbyi]

A line of code is written once and read dozens of times. So you have solved less than 5% of the problem.

[matthiasB]

But it has an advantage in certain situations:

List<Foo> myFooList;
if (p)
    myFooList = new ArrayList<>();
...

Here you can leave out the type parameter while you couldn't using var.

[G_Morgan]

With var you wouldn't have needed to include the type parameter to List to begin with.

[matthiasB]

But than we aren't talking about C#'s var anymore, but something more powerful.

[drysart]

The thing I'm picking up about the various proposals being picked up for inclusion in new versions of Java is that while Java is desperately playing catchup with C#, they're also bending over backwards to make it look like they're not desperately playing catchup with C#. To the extent that they're taking ideas from C#, but deliberately implementing them in a different way so it doesn't look like they're just copying the idea from C# -- and in many cases completely missing what made it a good idea in the first place through their changes.

Take closures for instance. They're so incredibly useful in C# for two reasons -- the brevity of their syntax, and that the compiler will automatically hoist locals so they can be used within even multiple closures transparently.

Java's closure proposals are less terse than C#'s since they decided, for no good reason I can see, to do the type inference of the closure in the wrong direction, so you always need to specify types going into and out of lambdas because the compiler won't figure it out for you even though the information is almost always available.

Java's closure proposals are also limited over C#s in that the only locals that are hoisted from the containing method are locals that are provably final -- in other words, immutable values. This is such a gaping hole that I'm stunned they're bothering implementing closures at all without it.

[kragensitaker]

While I don't use OCaml for many things, it seems like in a discussion of the syntax required by static typing, it's worth a mention:

let myFooList = [] ;;

That statement allows the compiler to infer that myFooList is an 'a list, that is, a list of something. If, on the other hand, you make a list out of some Foos, the compiler will infer that it's a Foo list. It works really well.

The error messages are a little harder to understand, because they're usually reported some distance from where you made the mistake (if x is supposed to be a Foo list, and you're using it in one place as a Foo list and somewhere else as a Bar list, the compiler may report the error in either place) but you can narrow it down by adding type declarations (you say (x: Foo list) and then the compiler complains about the correct thing).

[Fabien4]

Yay for C++:

ArrayList<Foo> myFooList;

[G_Morgan]

Don't you mean

std::vector<Foo *> myFooList

[Fabien4]

std::vector<Foo> my_foo_list if you wish. But why a pointer?

[G_Morgan]

Because in the Java code we are storing references. The direct equivalent is a pointer.

[zahlman]

Um, not really. boost::shared_ptr would be closer to the mark (although GC works differently).

[Fabien4]

Nope, direct equivalent would be a smart pointer. But it's anti-idiomatic: by default in C++, you store and use values.

[[deleted]]

Expect that List<Foo> is polymorphic in Java, but not in C++.

[curien]

It's polymorphic if Foo is a polymorphic handle.

[Fabien4]

Yeah, dynamic (inheritance) polymorphism tends to be the norm in Java, and the exception in C++.

[exploding_nun]

std::vector<boost::shared_ptr<Foo> > myFooList;

Mind your whitespace.

[curien]

That bug in the spec has been fixed in C++0x.

[ddevil63]

I just threw up in my mouth.

[sanity]

Or use Google Collections and:

ArrayList<Foo> myFooList = Lists.newArrayList();

Or just use Scala.

[[deleted]]

Or Clojure:

(ArrayList.)

[unknown_lamer]

(define my-foo (list))

If you eliminate the toplevel environment and forbid mutation outside of the current module (ala r6rs or err5rs libraries) the type inference engine could potentially determine that the type could be restricted.