Ruby1.9 block scope

ruby-talk
21

Daniel DeLorme wrote:

Mike Gold wrote:

This was the 'tall' case, but there is also the 'wide' case of deeply
nested scopes. An equal helping of confusion applies here too.

One might argue that when 'x = 45' appears before bar and baz, this
_already_ changes the meaning of bar and baz. But the situation here is
entirely different, in my mind. Don't most of us read code from top to
bottom?

Yes, I would argue that, and I believe most of us don't *add* new code
always at the bottom of the file. If the bar and baz methods are defined
at the bottom and Joe Schmo adds x = 45 at the top, he has the same
problem. I really don't see how it's any different; it's symmetric. Bugs
happen, and it's possible to come up with "problems cases" for
absolutely every feature in ruby, but that doesn't mean they turn out to
be problems in reality.

I must ask: is this confusing to *you* or are you trying to "protect" a
hypothetical Joe Schmo? I'll take the hypothetically confused Joe Schmo
over the very *real* confusion of every ruby newbie who wonders why his
local var has disappeared after the loop.

Joe Schmo is all of us. Like D. Black says, all of us would need to
read the code twice before understanding it. In practice it is probably
more common to read with backtracking, which is even slower.

On the other hand, a ruby newbie who first encounters the rules for
local variable bindings (in the form of an error) should in no manner be
shielded from understanding them. That error is an important learning
experience.

For old-timers like me who, coming from lisp, often use closures to
preserve state in conjunction with define_method on anonymous classes,
the impact of the proposed change is significant (or unmanageable,
depending on your point of view). It transforms code complexity from
O(n)--n nested blocks--to O(n^2)--n nested blocks (which can't even be
called "nested" anymore) with backtracking.

···

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22

Daniel DeLorme wrote:

David A. Black wrote:

before you could begin to understand any of it. And you would have to
do that for *every* block, just in case, even if only 1% of them had a
variable defined later.

You dont need a two-pass visual scan; it just allows you to assume that
a variable defined a line L of a method will be available for reference
at line L+n, no matter if it was defined inside a block or not. It
should be common sense, really.

You have described exactly what D. Black and I mean by a two-pass scan.
Line L is dependent on code arbitrarily far ahead at L+n.

When you encounter the variable at line L+n, you should backtrack to
line L to determine whether or not the block scopes are aligned in such
a way to change the meaning of line L.

···

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23

I hope I'm not belaboring the point, but --

a = Class.new {
  define_method(:f) {
    x
  }

  define_method(:x) {
    33
  }
}.new

x = 44

b = Class.new {
  define_method(:f) {
    x
  }

  define_method(:x) {
    55
  }
}.new

p a.f # => 33
p b.f # => 44

You want a.f == 44. I do not.

If you think this is a contrived example then you haven't been exposed
to the elegant solution this gives for certain cases. At run-time we
are creating additional layers of abstraction which translate the
current context into pieces which are passed to lower levels.

···

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24

Hi,

···

In message "Re: ruby1.9 block scope" on Tue, 30 Sep 2008 17:24:03 +0900, Mike Gold <mike.gold.4433@gmail.com> writes:

The current ruby doesn't, but a future one might? The reason I posted
was because the phrase "I haven't introduced it yet" might imply that it
could happen.

Yes, it could happen, but not for sure. This idea revisit me
everytime I see "undefined local variable or method" error.

              matz.

25

Hi,

I hope I'm not belaboring the point, but --

<snip>

Yes, but

* Class.new and define_method is a rather rare examples, of which
   non-block counterpart introduce new scopes.
* In 1.9, you can have explicit block local variables if you expect
   any confusion.

              matz.

···

In message "Re: ruby1.9 block scope" on Wed, 1 Oct 2008 02:06:38 +0900, Mike Gold <mike.gold.4433@gmail.com> writes:

26

Yukihiro Matsumoto wrote:

Yes, but

* Class.new and define_method is a rather rare examples, of which
   non-block counterpart introduce new scopes.

But it's not a rare example for me. It's a great solution to an
otherwise tangled problem. When I have more time I can post some code
if my example is not convincing.

The whole point is that I don't want to introduce a new scope. As I
briefly mentioned, I am writing adapters which put a "new face" on the
current binding so that other levels can understand it.

By contrast, it would be convoluted to create all new scopes each with
instance variables pointing back to the places I just came from. The
code would be Java-like: reams of scaffolding which serve no end except
to compensate for a missing language feature.

* In 1.9, you can have explicit block local variables if you expect
   any confusion.

I would use the block local syntax every time, because I would always
expect confusion. The ruby "good feeling" would be gone, for me.
Effectively it would be Lisp "let" constructs, which is fine I guess,
but it's Lisp.

I have always thought of ruby as a (non-defmacro) Lisp "optimized" to
make programmers more happy. In this case I feel the proposed
"optimization" goes too far, producing unexpected behavior like in the
example I gave. But I realize POLS refers to your surprise, not my
surprise.

···

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27

Hi,

···

In message "Re: ruby1.9 block scope" on Wed, 1 Oct 2008 03:40:08 +0900, Mike Gold <mike.gold.4433@gmail.com> writes:

* Class.new and define_method is a rather rare examples, of which
   non-block counterpart introduce new scopes.

But it's not a rare example for me. It's a great solution to an
otherwise tangled problem. When I have more time I can post some code
if my example is not convincing.

<snip rest>

Thank you for your valuable input. And don't worry I have not decided
to put this in the language, even after 2.0.

              matz.

28

Mike Gold wrote:

Yukihiro Matsumoto wrote:

Yes, but

* Class.new and define_method is a rather rare examples, of which
   non-block counterpart introduce new scopes.

But it's not a rare example for me. It's a great solution to an otherwise tangled problem. When I have more time I can post some code if my example is not convincing.

Yes, I would love to see some non-contrived example code. I don't use define_method very often and I have hardly ever used Class.new, certainly not with several levels of nested blocks. The common case for which I *would* like to use this suggested featured is something like this:

1 collection.each do |element|
2 changed = true if element.change
3 end
4 do_something if changed

Are you really saying that upon reading line 4 you have to backtrack to line 2 in order to understand this code?

Far from a monster of O(n^2) complexity, this is the kind of code that most programmers would expect to work. Except it doesn't, and if I have to hazard a guess I would say that 90% of ruby newbies have run into that gotcha.

Of course we all learn to work around it and even exploit it, but it doesn't mean it was right to begin with. While "intuitive" and "POLS" are loaded terms, if 90% of newbies bump into a gotcha, IMHO it's the sign of a design flaw, and matz was right to notice it.

···

--
Daniel

29

Hi,

At Wed, 1 Oct 2008 03:53:26 +0900,
Yukihiro Matsumoto wrote in [ruby-talk:316501]:

Thank you for your valuable input. And don't worry I have not decided
to put this in the language, even after 2.0.

How about making all local variables other than block arguments
method/class/toplevel?

···

--
Nobu Nakada

30

Daniel DeLorme wrote:

1 collection.each do |element|
2 changed = true if element.change
3 end
4 do_something if changed

Are you really saying that upon reading line 4 you have to backtrack to
line 2 in order to understand this code?

I think the point is more this:

class Foo
  attr_reader :changed

  def update
    collection.each do |element|
      element << "\n" if changed # <<<<
    end
    ...
    ...
    ...
    collection.each do |element|
      changed = true if element.change
    end
  end
end

At the marked line, you can't tell whether 'changed' is a local variable
or a method call on this object until you have read forward to the end
of the method. For me, this is horrid.

Perhaps more importantly though, as far as I can see it makes irb pretty
useless, since it can't predict the future. What's supposed to happen
when you type this, one line at a time?

def tmp; "hello"; end
m = lambda { |x| tmp + x }
m[" world"]
tmp = "goodbye"
m[" world"]

This particular example may be somewhat contrived, but if code behaves
differently in irb than when read from a source file, that would be
horrid (and difficult to explain) too.

Regards,

Brian.

···

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31

Hello to all.

I wonder if someone is able to help me out. I'm using the RSCM gem to help us with our deployment and am running into some difficulties when using RSCM::Subversion.diff to get the changes made since a "base" revision.

When i call RSCM::Subversion.diff i get the following error:
*/
scm.diff("branches/optimization/Rakefile", 1533, 1665)/*

/svn: Can't write to stream: The pipe is being closed. c:/ruby/lib/ruby/gems/1.8/gems/rscm-0.5.1/lib/rscm/command_line.rb:111:in `verify_exit_code': (RSCM::CommandLine::ExecutionError)
dir : C:/Documents and Settings/Jim/My Documents/Work/OffroadDeployR
command : svn diff --revision 1533:1665 "svn://svn.******/Ruby/****/branches/optimization/Rakefile"
executed command : svn diff --revision 1533:1665 "svn://svn.******/Ruby/****/branches/optimization/Rakefile"
exitstatus: 1
STDERR TAIL START
doesn't exist
STDERR TAIL END
    from c:/ruby/lib/ruby/gems/1.8/gems/rscm-0.5.1/lib/rscm/command_line.rb:140:in `e'
    from c:/ruby/lib/ruby/gems/1.8/gems/rscm-0.5.1/lib/rscm/command_line.rb:70:in `execute'
    from c:/ruby/lib/ruby/gems/1.8/gems/rscm-0.5.1/lib/rscm/base.rb:281:in `execute'
    from c:/ruby/lib/ruby/gems/1.8/gems/rscm-0.5.1/lib/rscm/scm/subversion.rb:82:in `diff'
    from svn_wrapper.rb:30
    from svn_wrapper.rb:29:in `each'
    from svn_wrapper.rb:29
    from svn_wrapper.rb:28:in `each_pair'
    from svn_wrapper.rb:28
    from c:/ruby/lib/ruby/gems/1.8/gems/rscm-0.5.1/lib/rscm/revisions.rb:64:in `each'
    from c:/ruby/lib/ruby/gems/1.8/gems/rscm-0.5.1/lib/rscm/revisions.rb:64:in `each'
    from svn_wrapper.rb:7

/However if i copy the svn diff cmd from the error codeand run it from a command line, I get a diff of the files as expected.:

*/svn diff --revision 1533:1665 "svn://svn.******/Ruby/****/branches/optimization/Rakefile"/*
/
/I'm running Ruby 1.8.6 on Windows XP with the RSCM v0.5.1 (oh and Subversion 1.5.1). If i've missed anything let me know. Would really appreciate help if anyone can.

Cheers
Jim

32

Daniel DeLorme wrote:

Mike Gold wrote:

Yukihiro Matsumoto wrote:

Yes, but

* Class.new and define_method is a rather rare examples, of which
   non-block counterpart introduce new scopes.

But it's not a rare example for me. It's a great solution to an
otherwise tangled problem. When I have more time I can post some code
if my example is not convincing.

Yes, I would love to see some non-contrived example code. I don't use
define_method very often and I have hardly ever used Class.new,
certainly not with several levels of nested blocks. The common case for
which I *would* like to use this suggested featured is something like
this:

Because you've hardly ever used Class.new, and therefore are probably
not familiar with run-time generated classes which present a given
binding as different views to external onlookers, then you won't
appreciate what I have to say. To you everything is blub, and you
wonder why non-blub things are necessary. It's a paradox, that you need
to understand before you can understand. See
http://www.paulgraham.com/avg.html

The strategy I've described comes from Lisp. And it's awesome.

In fact I think my example does suffice. You just need to imagine it
being using on a slightly larger scale. As I said previously in this
thread, "it would be convoluted to create all new scopes each with
instance variables pointing back to the places I just came from. The
code would be Java-like: reams of scaffolding which serve no end except
to compensate for a missing language feature." Do you have a response
to that? Please don't repeat your argument yet again; instead, try to
understand what I've said.

1 collection.each do |element|
2 changed = true if element.change
3 end
4 do_something if changed

Are you really saying that upon reading line 4 you have to backtrack to
line 2 in order to understand this code?

Yes, I backtracked just now, before I read your last sentence above. I
checked to be sure that 'changed' means a local variable. I have to do
this with every variable in arbitrarily long scopes. You are thinking
in terms of small examples, but I am thinking about large ones.

Far from a monster of O(n^2) complexity,

It is O(n^2) complex because it requires a backtrack for each case. I
like to be certain what code does. I will always backtrack in order to
be certain.

this is the kind of code that
most programmers would expect to work. Except it doesn't, and if I have
to hazard a guess I would say that 90% of ruby newbies have run into
that gotcha.

You did not address my counterpoint to your argument here. You just
repeated your argument.

···

--
Posted via http://www.ruby-forum.com/\.

33

Nobuyoshi Nakada wrote:

How about making all local variables other than block arguments
method/class/toplevel?

On the plus side:

  def my_find(a)
    # res=nil not needed!
    a.each { |e| res = e if e =~ /foo/ }
    res
  end

On the minus side:

  m = lambda { |x,y| tmp = x*2; tmp + y }

Here tmp becomes a "static" variable (in C-speak) by default, unless you
write

  m = lambda { |x,y;tmp| ... }

I think this could cause hard-to-find bugs if the lambda is invoked from
multiple threads, and a lot of confusion for people expecting functions
written in this form to be side-effect free.

I guess it could become another difference between lambda and
Proc/block. I'm not sure if that would make things less confusing or
more.

I still think the current rule ("tmp is part of the closure if it is
assigned to earlier in the method") is much more sane than the suggested
alternative ("tmp is part of the closure if it is assigned to earlier
*or later* in the method")

···

--
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34

Daniel DeLorme wrote:

Yes, I would love to see some non-contrived example code. I don't use
define_method very often and I have hardly ever used Class.new,
certainly not with several levels of nested blocks. The common case for
which I *would* like to use this suggested featured is something like
this:

Because you've hardly ever used Class.new, and therefore are probably
not familiar with run-time generated classes which present a given
binding as different views to external onlookers, then you won't
appreciate what I have to say.

So Mike, please show us a real non-contrived example to enlighten us.
I've often used Class.new but still don't see your problem.

In fact I think my example does suffice.

No it doesn't. In your example there's no obvious need to use
Class.new and define_method.

Far from a monster of O(n^2) complexity,

It is O(n^2) complex because it requires a backtrack for each case. I
like to be certain what code does. I will always backtrack in order to
be certain.

You have to do that in current Ruby, too, because you have to check
for local variables that have been assigned to before the code blocks.
Expanding the checks to the end of the scope doesn't change the O
complexity.

Regards,
Pit

···

2008/10/1 Mike Gold <mike.gold.4433@gmail.com>:

35

Mike Gold wrote:

Because you've hardly ever used Class.new, and therefore are probably not familiar with run-time generated classes which present a given binding as different views to external onlookers, then you won't appreciate what I have to say. To you everything is blub, and you wonder why non-blub things are necessary. It's a paradox, that you need to understand before you can understand. See Beating the Averages

The strategy I've described comes from Lisp. And it's awesome.

Just because I've hardly ever *had* to use Class.new doesn't mean that I don't understand how it works or that I'm some kind of idiot. It's true that I don't appreciate what you have to say. And I imagine 99.9% of ruby coders won't either. The problem you describe is, by your own admission, rooted in using ruby as an alternative lisp. It's great that ruby is so flexible that you can do that, but ultimately ruby is ruby, not lisp. I wouldn't hold back the language just so that it can stay more like another language.

In fact I think my example does suffice. You just need to imagine it being using on a slightly larger scale. As I said previously in this thread, "it would be convoluted to create all new scopes each with instance variables pointing back to the places I just came from. The code would be Java-like: reams of scaffolding which serve no end except to compensate for a missing language feature." Do you have a response to that? Please don't repeat your argument yet again; instead, try to understand what I've said.

I apologize for repeating myself. In my defense, your argument is gibberish to me so there's not much I can reply to. If you're talking about added complexity in the interpreter, I fail to see how that should affect the language's design. And if you're talking about added complexity in the ruby code, I fail to see how the proposal has anything to do with instance variables or would require additional "scaffolding". At most you'd need to avoid re-using variable names in a given scope, and that good programming practice anyway.

Yes, I backtracked just now, before I read your last sentence above. I checked to be sure that 'changed' means a local variable. I have to do this with every variable in arbitrarily long scopes. You are thinking in terms of small examples, but I am thinking about large ones.
It is O(n^2) complex because it requires a backtrack for each case. I like to be certain what code does. I will always backtrack in order to be certain.

Ok, I now see that it is indeed O(n^2) complex. But you *already* have to do that for large scopes; you have to consider every variable that previously appeared in the scope, and that's O(n^2). The new rule would make it 2*O(n^2) [which, in math speak, is still O(n^2)]. But this is only a problem if n is very large; if you really have a scope that long then I dare say that you already have a problem and some refactoring is in order. Whatever happened to "no more than 10-20 lines per method" ?

···

--
Daniel

36

res = a.find { |e| e =~ /foo/ }

···

On 1 oct. 08, at 04:18, Brian Candler wrote:

Nobuyoshi Nakada wrote:

How about making all local variables other than block arguments
method/class/toplevel?

On the plus side:

  def my_find(a)
    # res=nil not needed!
    a.each { |e| res = e if e =~ /foo/ }
    res
  end

37

Brian Candler wrote:

  def my_find(a)
    # res=nil not needed!
    a.each { |e| res = e if e =~ /foo/ }
    res
  end

IMHO, the res=nil is necessary to preserve the semantics of a closure:

A lambda/closure should *enclose* the variables around it into a neat
package. It should *not* emit new variables into its surroundings!
Otherwise, it would be something like an "opener" instead of a
"closure".

I still think the current rule ("tmp is part of the closure if it is
assigned to earlier in the method") is much more sane than the suggested
alternative ("tmp is part of the closure if it is assigned to earlier
*or later* in the method")

I agree wholeheartedly and I hope Ruby does not change in this regard.

···

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38

Pit Capitain wrote:

So Mike, please show us a real non-contrived example to enlighten us.
I've often used Class.new but still don't see your problem.

Do you understand my previous paragraph:

By contrast, it would be convoluted to create all new scopes each with
instance variables pointing back to the places I just came from. The
code would be Java-like: reams of scaffolding which serve no end except
to compensate for a missing language feature.

I ask because I've seen no indication that others have gotten it besides
matz. A larger example would be just what I've written already, only
larger. But if we aren't first on the same page with respect to the
above, the larger example will not help either. There is nothing I'm
holding back. If it's not obvious then ask a question.

I would suggest to actively seek what is meant by the above, rather than
to passively demand code which will infuse you with enlightenment. It
won't.

Rather, treat it as a homework problem: construct a case where a set of
dynamically generated _unique_ classes bound to the current context
results in better code compared to a set of named classes defined in a
different scopes containing instance variables to that current context.
Go.

I don't mean to lecture, but after the necessary tools have been laid on
the table, I do resent students who demand to be shown. It is only the
curious who really learn. Questions are welcome. Demands are not.

You have to do that in current Ruby, too, because you have to check
for local variables that have been assigned to before the code blocks.

But you don't have to check for locals that have been assigned after.
See my example.

···

--
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39

Daniel DeLorme wrote:

Ok, I now see that it is indeed O(n^2) complex. But you *already* have to do that for large scopes; you have to consider every variable that previously appeared in the scope, and that's O(n^2). The new rule would make it 2*O(n^2) [which, in math speak, is still O(n^2)]. But this is only a problem if n is very large; if you really have a scope that long then I dare say that you already have a problem and some refactoring is in order. Whatever happened to "no more than 10-20 lines per method" ?

Sorry, I didn't think this through enough.

The new rule would not change the current rule that variables defined at L are not considered as variables at all at lines L-n. So under the new rule, code would have mathematically the *same* complexity but practically it might be more tricky:

   define_method :foo do
     #say, 20 lines of code
   end
   foobar

under new rules, you'd have to backtrack until the top of the scope to see if "foobar" is a variable or not.

under old rules, you'd have to backtrack until the top of the scope to see if "foobar" is a variable or not, while taking into account that variables defined in a sub-scope are self-contained, which means that you can skip large sections of indented code.

I hope I got it right this time.

···

--
Daniel

40

Joe Wölfel wrote:

···

On 1 oct. 08, at 04:18, Brian Candler wrote:

  end

res = a.find { |e| e =~ /foo/ }

Yes I know. That's why I called it "myfind". I made a simple concrete
example of a pattern: iterate doing each, assign something during the
iteration, use the assigned value outside of the iteration.

Enumerable#find by itself is not sufficient for all programming needs.
--
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