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Re: new Java lambda syntax

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On 13/09/2011 4:29 PM, Tom Anderson wrote:
> Basically, lambdas can be either method-like (they return values from
> themselves) or statement-like (they return values from their enclosing
> methods).

Smalltalk blocks can do both. The usual return operator ^foo returns foo 
from the enclosing method, but the if the block doesn't do a nonlocal 
return or exception throw of any kind it evaluates to a value which is 
equal to that resulting from the last expression inside of it. So:

[:a | ^(a + 1)] value: 3

returns 4 from the enclosing method;

[:a | (a + 1)] value: 3

returns 4 into the enclosing scope; and

[:a :b | b ifTrue: [^a] ifFalse: [a]] value: 3 value: x

returns 3 from the enclosing method if x is true and evaluates to 3 
otherwise.

Yes, this can be confusing. But blocks are used to implement ifs, loops, 
and other control structures so this ability seems necessary, 
particularly to have any way to break out of a loop early, say because 
you found something. It also means that Smalltalk methods and blocks are 
fundamentally different things, whereas the Java lambda proposal 
generates methods under the hood, rather than some new, separate 
freestanding-function category of thing.

> Lambdas are method-like in all the current dynamic languages
> (AFAIK - they certainly are in Smalltalk, Python and Javascript).

Um, not really; see above. Smalltalk blocks are objects, and the value:, 
value: value:, etc. methods can be called on them to run their code, but 
they aren't themselves methods. In particular, they are different 
classes in the Smalltalk object system: BlockContext vs. CompiledMethod.

If you meant though that they can evaluate to a value, that's true. On 
the other hand if you meant they can't also nonlocal-return, that's not 
true.

Incidentally, you can implement the behavior of Smalltalk blocks in even 
Java 1.1, with a class something like:

public class BlockReturn extends RuntimeException {
     public final Object value;
     public BlockReturn (x) { value = x; }
}

public interface Block {
     Object do (Object x);
}

...

public static Vector forEach (Vector i, Block b) {
     Vector res = new Vector();
     for (Enumeration e = v.elements(); e.hasMoreElements();) {
         res.addElement(b.do(e.nextElement()));
     }
     return res;
}

...

public Object nullOrHashCodes (Vector v) {
     try {
         return forEach(v, new Block() { public Object do (Object x) {
             if (x == null) throw new BlockReturn(null);
             return Integer.valueOf(x.hashCode());
         }}
     } catch (BlockException b) { return b.value; }
}

That last should end up taking a vector and returning null if it 
contains nulls and otherwise a vector of Integers equal to the hash 
codes, in order, of the objects in the input vector. Obviously, it's not 
idiomatic Java 1.1, let alone idiomatic Java SE 6 where you'd just use a 
foreach loop over a List, but it is isomorphic to how you'd do something 
like that in Smalltalk, where it would be something like

coll collect: [:x | x ifNil: [^nil] ifNotNil: [x hash]]

> The rationale was that enclosing any given wodge of code in a lambda
> definition immediately followed by an invocation of that lambda would
> not change the behaviour of the code. This seemed dubious to me, and
> would have led to very weird programming experiences. It would, though,
> have enabled a wider range of funky control structures implemented on
> top of lambdas, which some people are very keen on.

Smalltalkers, probably. Possibly Lispers too, though in Lisp lambdas 
can't nonlocally return; Lispers implement new control structures using 
macros instead, at least in the case that they need short-circuiting 
behavior under some circumstances, such as loops in the bodies of which 
there's some kind of break statement analogue possible. But both 
Smalltalkers and Lispers are used to being able to implement new control 
structures inside of their language.

It's not the first thing they'd probably miss in Javaland, though; the 
lack of an interactive transcript would probably top the list, and the 
limited reflection/introspection capabilities would come a close second. 
It's easy to create new functions, methods, or classes on the fly at 
runtime, for example, in those but not in Java. In fact, this is related 
to Java's lack of an interactive transcript, in that that degree of 
runtime reflection capability is needed to implement such a transcript 
if it's to be possible to enter new code or bugfixes at it and not just 
run ad hoc tests of existing code. (Not to mention, Windows-based 
Lispers tend to use their Lisp repl as a command line shell in 
preference to Windows's crummy cmd.exe. But inability to define new 
functions at the repl would make it much less useful for such, since you 
couldn't write ad hoc scripts and thus couldn't do the equivalent of 
sed, awk, and perl hacking at it. For that matter, you'd lack lambdas 
too, since using a lambda at the repl implicitly defines a new function, 
so even doing something as simple as get a list of files in some 
directory, sort descending by size, and return that list would become 
impossible given that Lisp sort functions tend to expect the less-than 
relation to use to be provided via lambda and tend not to already have a 
built-in function that implements a less-than relation on files that is 
based on file size.)

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  Re: new Java lambda syntax Roedy Green <see_website@mindprod.com.invalid> - 2011-09-08 15:18 -0700
    Re: new Java lambda syntax Arne Vajhøj <arne@vajhoej.dk> - 2011-09-08 18:27 -0400
    Re: new Java lambda syntax BGB <cr88192@hotmail.com> - 2011-09-08 15:40 -0700
      Re: new Java lambda syntax Arne Vajhøj <arne@vajhoej.dk> - 2011-09-08 19:27 -0400
        Re: new Java lambda syntax BGB <cr88192@hotmail.com> - 2011-09-08 16:29 -0700
          Re: new Java lambda syntax Arne Vajhøj <arne@vajhoej.dk> - 2011-09-08 19:48 -0400
          Re: new Java lambda syntax Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2011-09-08 17:56 -0700
            Re: new Java lambda syntax BGB <cr88192@hotmail.com> - 2011-09-08 22:23 -0700
    Re: new Java lambda syntax "Nasser M. Abbasi" <nma@12000.org> - 2011-09-08 16:41 -0700
    Re: new Java lambda syntax Joshua Cranmer <Pidgeot18@verizon.invalid> - 2011-09-08 20:50 -0500
  Re: new Java lambda syntax bugbear <bugbear@trim_papermule.co.uk_trim> - 2011-09-09 09:33 +0100
  Re: new Java lambda syntax Tom Anderson <twic@urchin.earth.li> - 2011-09-10 14:48 +0100
    Re: new Java lambda syntax Steven Simpson <ss@domain.invalid> - 2011-09-10 16:17 +0100
      Re: new Java lambda syntax Tom Anderson <twic@urchin.earth.li> - 2011-09-11 13:06 +0100
        Re: new Java lambda syntax Steven Simpson <ss@domain.invalid> - 2011-09-11 17:18 +0100
          Re: new Java lambda syntax BGB <cr88192@hotmail.com> - 2011-09-11 11:08 -0700
            Re: new Java lambda syntax Steven Simpson <ss@domain.invalid> - 2011-09-11 20:14 +0100
              Re: new Java lambda syntax BGB <cr88192@hotmail.com> - 2011-09-11 14:08 -0700
              Re: new Java lambda syntax Joshua Cranmer <Pidgeot18@verizon.invalid> - 2011-09-11 17:07 -0500
                Re: new Java lambda syntax BGB <cr88192@hotmail.com> - 2011-09-11 16:18 -0700
                Re: new Java lambda syntax Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid> - 2011-09-13 02:00 +0200
                Re: new Java lambda syntax Arne Vajhøj <arne@vajhoej.dk> - 2011-09-12 20:59 -0400
                Re: new Java lambda syntax Joshua Cranmer <Pidgeot18@verizon.invalid> - 2011-09-12 21:24 -0500
                Re: new Java lambda syntax Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid> - 2011-09-13 21:04 +0200
                Re: new Java lambda syntax Joshua Cranmer <Pidgeot18@verizon.invalid> - 2011-09-13 15:22 -0500
                Re: new Java lambda syntax Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid> - 2011-09-13 23:25 +0200
                Re: new Java lambda syntax Tom Anderson <twic@urchin.earth.li> - 2011-09-13 21:29 +0100
                Re: new Java lambda syntax Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid> - 2011-09-13 23:26 +0200
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                Re: new Java lambda syntax Tom Anderson <twic@urchin.earth.li> - 2011-09-15 21:58 +0100
                Re: new Java lambda syntax Joshua Cranmer <Pidgeot18@verizon.invalid> - 2011-09-13 21:36 -0500
                Re: new Java lambda syntax supercalifragilisticexpialadiamaticonormalizeringelimatisticantations <supercalifragilisticexpialadiamaticonormalizeringelimatisticantations@averylongandannoyingdomainname.com> - 2011-09-13 22:49 -0400
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                Re: new Java lambda syntax supercalifragilisticexpialadiamaticonormalizeringelimatisticantations <supercalifragilisticexpialadiamaticonormalizeringelimatisticantations@averylongandannoyingdomainname.com> - 2011-09-14 03:01 -0400
                Re: new Java lambda syntax "winkleMeister" <..00@00.00.00.1> - 2011-09-14 09:59 +0000
                Re: new Java lambda syntax supercalifragilisticexpialadiamaticonormalizeringelimatisticantations <supercalifragilisticexpialadiamaticonormalizeringelimatisticantations@averylongandannoyingdomainname.com> - 2011-09-15 10:16 -0400
                Re: new Java lambda syntax supercalifragilisticexpialadiamaticonormalizeringelimatisticantations <supercalifragilisticexpialadiamaticonormalizeringelimatisticantations@averylongandannoyingdomainname.com> - 2011-09-14 06:40 -0400
                Re: new Java lambda syntax supercalifragilisticexpialadiamaticonormalizeringelimatisticantations <supercalifragilisticexpialadiamaticonormalizeringelimatisticantations@averylongandannoyingdomainname.com> - 2011-09-15 10:16 -0400
                Re: new Java lambda syntax lightworker <etts@0n.org.null> - 2011-09-16 01:57 +0000
                Re: new Java lambda syntax thoolen <th00len@th0lenbot.thorium> - 2011-09-15 22:41 -0400
                Re: new Java lambda syntax Arved Sandstrom <asandstrom3minus1@eastlink.ca> - 2011-09-14 06:29 -0300
                Re: new Java lambda syntax BGB <cr88192@hotmail.com> - 2011-09-14 07:40 -0700
                Re: new Java lambda syntax Lew <lewbloch@gmail.com> - 2011-09-14 08:01 -0700
                Re: new Java lambda syntax BGB <cr88192@hotmail.com> - 2011-09-14 14:50 -0700
                Re: new Java lambda syntax Lew <lewbloch@gmail.com> - 2011-09-14 18:02 -0700
                Re: new Java lambda syntax BGB <cr88192@hotmail.com> - 2011-09-14 21:08 -0700

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