Ruby doesn't implement x++ for Fixnum's because?

Hi,

In The Well Grounded Rubyist (which, along with The Ruby Way, I
love), Dave Black presents a justification for the x++ omission in
Ruby. I thought I’d see whether I could implement just the
incrementing portion of C’s x++ functionality (without the prefix/
postfix issues) in pure Ruby. I wasn’t willing to delve into Matz’
implementation of higher-level Ruby functions or low-level C/C++
code. Below is what I came up with.

Q1. Test 1 failed, I imagine, because the interpret thought “Ah, 7’s
a Fixnum which I ‘ve seen billions of times, so I know what to do with
it.” Thus, it ignored my over-riding definition of Fixnum. So I
call it “compiler error”, to use the vernacular, by reporting that x
had no method “pp”. Am I all wet?

Test 2 passed, but it with a couple of anomalies:

Q2. x.to_s yielded x’s class and address, suggesting x had become a
“semi-Fixed Num”, i.e, x now stored a REFERENCE to the to x’s data,
rather than the immediate storage of the data, as interpreter is wont
to do with Fixnum’s. Am I all wet?

Q3. y.inspect yielded only it’s value (8) and nothing else,
suggesting the x.pp yielded a full-fledged Fixnum. Am I all wet?

Dave Black argues that since Fixnum values use immediate storage
rather than referenced storage, x=1;x++ would be akin to changing 1
into 2. This example somewhat supports that view, but the
interpreter sidestepped the matter by creating a reference for x’s
value and an immediate value for y (=x.pp).

Any ideas,
Richard

class FixNum
attr :val
def initialize (fixnum)
puts “Initializing object %d” % fixnum
raise “Initializer not a Fixnum” if
fixnum.class != Fixnum
puts “\nInitializing Fixnum to %d” % fixnum
@val = fixnum
end

def pp
@val+=1
end
end

Test 1 (Failure)

x = 7; y = x.pp; z =x.class # undefined method `pp’ for 7:Fixnum
(NoMethodError)

Test 2 (Success)

x = FixNum.new(7); y = x.pp; z =x.class
puts [x.to_s, y.inspect, z.to_s].join("; ") # => FixNum:0x2b62694;
8; FixNum

Hi –

On Fri, 30 Oct 2009, RichardOnRails wrote:

Hi,

In The Well Grounded Rubyist (which, along with The Ruby Way, I
love),

Thanks :slight_smile:

call it “compiler error”, to use the vernacular, by reporting that x
had no method “pp”. Am I all wet?

x is a FixNum, not a Fixnum. In fact you’re not actually reopening
Fixnum; you’ve created an entirely new, unrelated class called FixNum.

Is that what you meant to do? (I’ll hold off discussing your other
questions further pending clarification of that :slight_smile:

David


The Ruby training with D. Black, G. Brown, J.McAnally
Compleat Jan 22-23, 2010, Tampa, FL
Rubyist http://www.thecompleatrubyist.com

David A. Black/Ruby Power and Light, LLC (http://www.rubypal.com)

On Oct 29, 9:18 pm, RichardOnRails
[email protected] wrote:

Q1. Test 1 failed, I imagine, because the interpret thought “Ah, 7’s
to do with Fixnum’s. Am I all wet?
Any ideas,
end

Test 2 (Success)

x = FixNum.new(7); y = x.pp; z =x.class
puts [x.to_s, y.inspect, z.to_s].join("; ") # => FixNum:0x2b62694;
8; FixNum

Hi,

Please ignore my questions. I just discovered a major mistake: I
thought I was overriding Ruby’s Fixnum class, but I misspelled it at
FixNum. That error invalidates most if not all of the things I
reported. Mea Culpa.

I’ll correct my errors and probably have a new theory subsequently.

Best wishes,
Richard

2009/10/30 RichardOnRails
[email protected]:

I’ll correct my errors and probably have a new theory subsequently.

IMHO the story goes like this: absence of postfix ++ and – is a
consequence of the fact that numeric types are immutable in Ruby which
makes an assignment necessary for these operators. Although that
would be doable, it would not immediately be obvious when looking at
“foo++”. On the other side, “foo += 1” makes the assignment obvious
while still being pretty concise (you do not have to write "foo = foo

  • 1").

Which brings us to the question: what is the advantage of immutable
numbers? First, it avoids errors that could be caused by aliasing
(two objects refer the same object, one of them changes it, it changes
for the other one as well without being expected). Then, it is quite
natural if you consider mathematical numbers: they cannot change.

Additionally, the expression “1” can always refer to the same object
(in reality it’s a bit different but from a Ruby programmer’s
perspective the difference is not noticeable) which in fact makes
using numeric constants very efficient (as opposed to the expression
“‘foo’” which constructs a new String instance on every invocation,
albeit with a shared char array underneath which eases the pain a
bit).

This in turn makes integer math pretty efficient because if numbers
were mutable Ruby would have to create a new object for every result
of an operator evaluation. I am not saying that Ruby is ideal for
number crunching but it could be significantly slower if certain
design decisions would have been made differently.

Kind regards

robert

IMHO the story goes like this: absence of postfix ++ and – is a
consequence of the fact that numeric types are immutable in Ruby which
makes an assignment necessary for these operators.

You could “decorate” it with such a method though. This could look
something like this:

class FixNum < BasicObject

    def initialize(fn)
        @fn = fn
    end

    def succ!
        @fn += 1
    end

    def method_missing(*args, &block)
        @fn.send(*args, &block)
    end

end

a = FixNum.new(1)
a.succ!
p a
=> 2

Whether this is good practice or marginally useful …

On Fri, Oct 30, 2009 at 1:51 PM, lith [email protected] wrote:

       @fn = fn
   end

   def succ!
       @fn += 1
   end

   def method_missing(*args, &block)
       @fn.send(*args, &block)
   end

Hi lith,

can you extol your knowledge on me about what method_missing is doing?
I get what *args and &block is, but not quite sure what’s going on?

end

a = FixNum.new(1)
a.succ!
p a
=> 2

Whether this is good practice or marginally useful …


Kind Regards,
Rajinder Y.

http://DevMentor.org

Do Good! - Share Freely, Enrich and Empower people to Transform their
lives.

Rajinder Y. wrote:
[…]

can you extol

(Not what that word means.)

your knowledge on me about what method_missing is doing?

http://www.ruby-doc.org/docs/ProgrammingRuby/html/ref_c_object.html#Object.method_missing

Best,

Marnen Laibow-Koser
http://www.marnen.org
[email protected]

On Fri, Oct 30, 2009 at 11:05 AM, Robert K.
[email protected] wrote:

  • 1").
    I think that theres a more fundamental problem with ++ in a language
    like ruby, which has to do with the difference between objects and
    variables.

The c ++ and – operators change a variable, NOT a value.

so in C

a = 1
b = a
a++

a is now 2, but b is still 1.

Now, consider not immutable objects, but defining ++ for a mutable
object. I’ve named the method plus_plus instead of ++ since I can do
the former, but not the latter.

class String
def plus_plus
self << " plus a plus"
end
end

a = “A non-plussed string”
b = a

puts “a is #{a.inspect}”
puts “b is #{b.inspect}”

a.plus_plus

puts “a is #{a.inspect}”
puts “b is #{b.inspect}”

When we run this we see that the result is:

a is “A non-plussed string”
b is “A non-plussed string”
a is “A non-plussed string plus a plus”
b is “A non-plussed string plus a plus”

Because in languages like Ruby with object reference variable
semantics, methods can only operate on objects, not the variables
which reference them.

So if you COULD successfully define Fixnum#++, or Fixnum#plus_plus:

a = 1
a++

could only change the singleton instance of 1 into 2, which probably
isn’t something you’d really want to do.

By ‘you’ I’m not aiming at you Robert.

This reminds me of a very hard bug I encountered years ago when I was
first was learning to program in Fortran. In the original Fortran you
could assign a new value to a subroutine parameter inside the
subroutine. If the actual parameter value was an integer literal, you
could literally change 1 to 2, and that’s why I know you probably
don’t want to do this.

Rick DeNatale

Blog: http://talklikeaduck.denhaven2.com/
Twitter: http://twitter.com/RickDeNatale
WWR: http://www.workingwithrails.com/person/9021-rick-denatale
LinkedIn: http://www.linkedin.com/in/rickdenatale

On 10/30/2009 08:03 PM, Rick DeNatale wrote:

while still being pretty concise (you do not have to write "foo = foo

  • 1").

I think that theres a more fundamental problem with ++ in a language
like ruby, which has to do with the difference between objects and
variables.

The c ++ and – operators change a variable, NOT a value.

How their behavior is defined is completely up to the language designer.
You could make “a++” syntactic sugar for

(_tmp = a; a = a + 1; _tmp)

in the same way as “a += 1” is syntactic sugar for “a = a + 1” or “a ||=
10” is syntactic sugar for “a || a = 10”. In fact the sequence in
brackets above would be a completely reasonable way to do it in Ruby:
you could use “a++” in a similar way as in C++ and Java without
sacrificing Fixnum’s immutability.

But: you would hide the assignment in the syntactic sugared version and
this could lead to unexpected behavior, like

a = 1
b = a
a++

now why is b still 1???

For someone unaware of the hidden assignment but aware of the fact that
the object referenced by a must be mutable to make this work it would be
surprising that b is not changed as well. You could only discover that
by looking at #object_id of both (or using #equal?).

so in C

a = 1
b = a
a++

a is now 2, but b is still 1.

Yes - and in C++ you can have it otherwise:

[email protected]:~$ g++ x.cc && ./a.out
a++=1
a=2
b=2
[email protected]:~$ cat x.cc

#include <stdio.h>

int main( int argc, const char* argv[] )
{
int a = 1;
int &b = a;
printf(“a++=%d\n”, a++);
printf(“a=%d\n”, a);
printf(“b=%d\n”, b);
return 0;
}

[email protected]:~$

So if you COULD successfully define Fixnum#++, or Fixnum#plus_plus:

a = 1
a++

could only change the singleton instance of 1 into 2, which probably
isn’t something you’d really want to do.

Not necessarily. If this was implemented in the language the sequence
“1” would be an object constructor in the same way as string “constants”
are.

[email protected]:~$ ruby1.8 -e ‘2.times { puts “foo”.object_id }’
-605970738
-605970758

That would the only reasonable way to do it. Then there would be no
singleton instance which in turn would have the said performance
implications.

I would consider the example from Fortran that you presented (changing
the value of the constant 1) rather a bug either in the language
implementation or in the design. This does not mean that other
languages would have to repeat that. :slight_smile:

Kind regards

robert

int main( int argc, const char* argv[] )
{
int a = 1;
int &b = a;
printf(“a++=%d\n”, a++);
printf(“a=%d\n”, a);
printf(“b=%d\n”, b);
return 0;
}

No, you can’t:

$ ./ab
a: 2 @ 0xbfe413f8
b: 2 @ 0xbfe413f8

$ cat ab.cc
#include

int main(void) {
int a = 1;
int &b = a;
a++;
std::cout << "a: " << a << " @ " << &a << std::endl;
std::cout << "b: " << b << " @ " << &b << std::endl;
return 0;
}

There is only one variable, b is just another name for it.

mfg, simon … l

On Fri, Oct 30, 2009 at 6:20 PM, Robert K.
[email protected] wrote:

On 10/30/2009 08:03 PM, Rick DeNatale wrote:

How their behavior is defined is completely up to the language designer.
You could make “a++” syntactic sugar for

(_tmp = a; a = a + 1; _tmp)

in the same way as “a += 1” is syntactic sugar for “a = a + 1” or “a ||= 10”
is syntactic sugar for “a || a = 10”. In fact the sequence in brackets
above would be a completely reasonable way to do it in Ruby: you could use
“a++” in a similar way as in C++ and Java without sacrificing Fixnum’s
immutability.

And I would argue that you still can’t implement ++ for fixnum since
there isn’t a ++ method, any more than there is a += method.

But: you would hide the assignment in the syntactic sugared version and this
could lead to unexpected behavior, like

Which I think gets to the fundamental difference.

In C/C++ an integer variable is just a bit-string of a particular
length interpreted as a signed (probably 2s complement) integer, and
++ is a operator on the variable which twiddles those bits.

In Ruby and languages of its ilk, a variable is a reference to an
object, and can only be modified by an assignment, not by a method of
the object it happens to be referencing at the moment, whether that
object is mutable or not.


Rick DeNatale

Blog: http://talklikeaduck.denhaven2.com/
Twitter: http://twitter.com/RickDeNatale
WWR: http://www.workingwithrails.com/person/9021-rick-denatale
LinkedIn: http://www.linkedin.com/in/rickdenatale

On Sat, Oct 31, 2009 at 1:35 PM, Simon K. [email protected] wrote:

#include <stdio.h>

int a = 1;
int &b = a;
a++;
std::cout << "a: " << a << " @ " << &a << std::endl;
std::cout << "b: " << b << " @ " << &b << std::endl;
return 0;
}

There is only one variable, b is just another name for it.

I’m not going to run the experiment, but I believe that if the

int &b = a;

were changed to

int b = a;

Then the result would be the same as in Robert’s ‘surprising’ ruby
example.

Which only points out the different in meaning between a C ‘reference’
variable, which aliases another variable, and the concept of a ruby
variable as a reference to an object.

And no, I don’t think that adding syntactic sugar to Ruby to make a++
the same as either

a += 1
or
a = a.succ

is a good idea.

As a matter of fact, neither of this is REALLY the same as a++, since
things like

b = a++

should assign the ORIGINAL value of a to b, as opposed to

b = ++a

Things like post and pre increment/decrement operators in C like
languages are best reserved to languages with the same basic
structure.

To my mind they are akin to the different rules for gender, and
agreement in different families of languages.


Rick DeNatale

Blog: http://talklikeaduck.denhaven2.com/
Twitter: http://twitter.com/RickDeNatale
WWR: http://www.workingwithrails.com/person/9021-rick-denatale
LinkedIn: http://www.linkedin.com/in/rickdenatale

On Oct 30, 11:05 am, Robert K. [email protected] wrote:

  • 1").
    using numeric constants very efficient (as opposed to the expression
    Kind regards

robert


remember.guy do |as, often| as.you_can - without endhttp://blog.rubybestpractices.com/

Hi Robert,

Thank you for your thoughtful and extensive responses. I apologized
earlier for my sloppy analysis. I hope the following offers higher
quality.

(a) You say, in part:
“… immutable numbers? First, it avoids errors that could be caused
by aliasing (two objects refer the same object, one of them changes
it, it changes for the other one as well without being expected”
(b) Dave Black, in his truly excellent “The Well-Grounded Rubyist”
says, in part:
“Any object that’s represented as an immediate value is always exactly
the same object” and
“The reason [there’s no x=1; x++] is that , due to the immediate
presence of 1 in x, means you’d be changing 1 into 2,
and that makes no sense”

  1. I’d rather discuss this matter in concrete terms rather than
    abstractions. Please look at the following method (to generate
    results) and two statements:

def show(v)
“Got #{v}, class = #{v.class}, object_id = #{v.object_id}
(v.object_id-1)/2 = #{(v.object_id-1)/2 }”
end

a = 1; show (a) => Got 1; class = Fixnum; object_id = 3; v >> 1
= 1
b = 1; show (b) => Got 1; class = Fixnum; object_id = 3; v >> 1
= 1
a == b => true

Both “a” and “b” have their respective values held as immediate values
(embedded in their object_id’s), but they are NOT the same thing
because they’re ultimately held in their respective entries in the
symbol table. So, object_id’s for Fixnum’s are synthetic. To think
that those object_id’s point to a location in a memory area that
stores the 32-bit 000…001 (in a 32-bit machine/OS) is to contradict
the meaning of immediacy and defeat the very efficiency that immediate
values offer.

  1. Now please consider the following supplement to the statements
    above:

a += 1; show (a) => Got 2; class = Fixnum; object_id = 5; v >> 1
= 2
show (a) => Got 2; class = Fixnum; object_id = 5; v >> 1 = 2
show (b) => Got 1; class = Fixnum; object_id = 3; v >> 1 = 1

The assignment of “a += 1” to “a” changed a’s object_id to embed a new
value: 2. Despite that change of 1 to 2 in a’s object, “b” remains
set to 1. “b” did not suffer the calamity of a universal change of
all Fixnum 1’s to 2’s.

  1. Conclusion:

a += 1 is equivalent to a++’s natural meaning. In fact, we further
supplement the above statements with:

class Fixnum
def pp # We can’t define ++ because of a compiler restriction.
self + 1
end
end

a=1; show(a.pp) => Got 2; class = Fixnum; object_id = 5; v >> 1
= 2
show(b) => Got 1; class = Fixnum; object_id = 3; v >> 1 = 1

Furthermore, pp works fine on the boundary condition for Fixnum’s on a
common 32-bit cpu/OS, i.e. 2**30-1:

a = 230-1; show (a) => Got 1073741823; class = Fixnum; object_id
= 2147483647; v >> 1 = 1073741823
a = 2
30; show (a) => Got 1073741824; class = Bignum; object_id =
22737670; v >> 1 = 11368835

So x++ works fine in the form of pp for positive Fixnum’s. I assume
it’ll work fine for non-positives, also… A compiler change to allow
“def ++” is necessary to finally add ++ to Fixnum.

Again, thanks for your responses to my question.

Best wishes,
Richard

On Fri, Oct 30, 2009 at 12:03 PM, Rick DeNatale
[email protected]wrote:

I think that theres a more fundamental problem with ++ in a language
like ruby, which has to do with the difference between objects and
variables.

Personally I see nothing interesting about the behavior of ++ in any
mutable
state language.

Now, consider not immutable objects, but defining ++ for a mutable
object. I’ve named the method plus_plus instead of ++ since I can do
the former, but not the latter.

How is ++ any different from << except for << taking an argument? (given
hypothetical C-like ++ behavior)

There is already extensive precedent in Ruby for destructive method
calls
that mutate state, and they all lead to the same confusion.

There is nothing interesting with Ruby in this regard, except that Ruby
does
seem to go out of its way to do things immutably by default, which, in
my
opinion, is pretty cool. But in the end Ruby is still very much a
mutable
state language.

On Fri, Oct 30, 2009 at 3:20 PM, Robert K.
[email protected] wrote:

consequence of the fact that numeric types are immutable in Ruby which
The c ++ and – operators change a variable, NOT a value.
immutability.
I think it would make more sense, if you were going to define a “++”
syntax for Ruby, to use “a.succ” rather than “a+1”, and if you are
going to create syntax which does an assignment but returns the
pre-assignment value of the variable receiving the assignment, I’d go
further and include a more general form for that, say “:=”, such that
“a := b” is equivalent to “(_ = a),(a = b), _” and then define postfix
“++” such that “a++” is “a := a.succ” and prefix “++” such that “++a”
is equivalent to “a = a.succ”.

On 2009-11-04, Rick DeNatale [email protected] wrote:

Why this wierd semantic? Because it the original use case was for
pointers rather than integers, and it goes back to C originating
originally as a kind of high level assembly language for the DEC
PDP-11 which had postincrement and predecrement addressing modes used
for stepping through strings or arrays.

Apparently apocryphal – the increment usage predates the PDP 11 port
of C. (Interestingly, there’s now a ton of sources repeating the story,
but so far as I know, the canonical answer from Ritchie and Thompson
was that the increment operator predates C proper, and thus the PDP 11
work.)

Because pre-ANSI C allowed easy spoofing/overlay of types, it also
worked for integers and was used in contexts like for loops
for(i = 0; i < max;i++)

It is not because of spoofing or overlay of types that this worked,
but because the operator’s defined for all types.

If you want the real semantics of the C/C++ post-increment operator,
I’m afraid you’ll have to look for it in a C family language, not an
object reference semantic language like Ruby.

This part, though, I totally agree with. It’s not coherent to imagine a
++ operator, especially a postincrement, working in a Ruby-like
language.

-s

Tony A. wrote:

state language.

Now, consider not immutable objects, but defining ++ for a mutable
object. I’ve named the method plus_plus instead of ++ since I can do
the former, but not the latter.

How is ++ any different from << except for << taking an argument? (given
hypothetical C-like ++ behavior)

Maybe I am missing something but how would one print a variable and
increment it with the same statement? In C one would just write printf
("%i/n", x++). I haven’t seen any easy way to do this with Ruby. Using
<< or +=1 you have to have a separate statement for the assignment. So
instead of having something like:
i = 1
while (i < 10)
puts i++
end
you have to have:
i = 1
while (i< 10)
puts i; i +=1
end
I can see a reason for not being able to do 2++, but not i++.

On Tue, Nov 3, 2009 at 10:25 PM, RichardOnRails
[email protected] wrote:

a += 1 is equivalent to a++’s natural meaning. In fact, we further
supplement the above statements with:

class Fixnum
def pp # We can’t define ++ because of a compiler restriction.
self + 1
end
end

So x++ works fine in the form of pp for positive Fixnum’s. I assume
it’ll work fine for non-positives, also… A compiler change to allow
“def ++” is necessary to finally add ++ to Fixnum.

No x++ doesn’t work fine if you want it to act like C’s post-increment
operator.

Let’s leave Ruby aside for a moment and consider this snippet of C code.

int a = 1;
int b = a++

After these two lines a is 2, and b is 1, since a++ returns the
original value of a and then increments a.

Why this wierd semantic? Because it the original use case was for
pointers rather than integers, and it goes back to C originating
originally as a kind of high level assembly language for the DEC
PDP-11 which had postincrement and predecrement addressing modes used
for stepping through strings or arrays.

Because pre-ANSI C allowed easy spoofing/overlay of types, it also
worked for integers and was used in contexts like for loops
for(i = 0; i < max;i++)

where the value of the expression i++ was never used, only the side
effect that it left a as 1 higher than before was important.

Because of this, many C/C++ programmers seem to be unaware or forget
that the value of a++ is a, not (a+1). In fact Bjarne Stroustrup used
to rib himself by pointing out that the value of C++ was exactly the
same as C, because he IS certainly aware of the semantics.

Now what happens if we translate my initial snippet into Ruby using
your pp method

class Integer

Not need to restrict to FixNums is there

Name it pp since ++ isn’t a valid Ruby method selector

def pp
self + 1
end
end

a = 1
b = a.pp

puts “a is #{a}, b is #{b}”

Produces not “a is 2, b is 1” as it would if it correctly implemented
C language semantics but

a is 1, b is 2

just the opposite of what would be expected by a C/C++ programmer who
understood the meaning of post-increment ++

The fact of the matter is that you can’t write a ++ or a pp method in
Ruby which works like the C post-increment because it requires the
method to change the value of a variable which is bound to the object
and the method only knows what the object knows and the object doesn’t
know which variable(s) refer to it.

consider

a = b = c = 1
b.pp

There are at least 3 variables which refer to the Fixnum object 1, The
fact that that reference is through a special form object id computed
by multiplying 1 by 2 and adding 1, as it is in several Ruby
implementations, is irrelevant. So is the fact that Fixnums happen to
be immutable. No object knows what variables reference it, and
wouldn’t have a way to directly change those variables bindings
anyway.

The rebinding of a in

a = a.pp

doesn’t happen in the pp method it happens in the calling context
because of the assignment expression there.

So if you just want a ++ which returns the incremented value of a
fixnum then you can write it as pp, but you really don’t need to since
Integer#succ does just that.

If you want the real semantics of the C/C++ post-increment operator,
I’m afraid you’ll have to look for it in a C family language, not an
object reference semantic language like Ruby.


Rick DeNatale

Blog: http://talklikeaduck.denhaven2.com/
Twitter: http://twitter.com/RickDeNatale
WWR: http://www.workingwithrails.com/person/9021-rick-denatale
LinkedIn: http://www.linkedin.com/in/rickdenatale

Of course I had to jump in here.

Yes, a++ and ++a could easily be rewritten by the parser into the
appropriate increment+set of a and the expression either returns the
incremented value or the non-incremented value. And I would like to
see that added. It doesn’t fundamentally change the expectations of
the programmer, and it provides a one-character-shorter version of
a+=1. There’s really no reason it shouldn’t be added, because even in
Java or C, you are never modifying arbitrary references to that
value…you are always re-assigning the value a given variable
points to.

This example:

a = 1
b = a
a++

Would cause exactly the same results in every language I’ve worked
with…b would be 1 and a would be 2. The ++ operator never modifies a
value, it modifies what value the variable has assigned to it. If it
were modifying a value, then using ++ to bump a pointer through memory
offsets would have horrible side effects for anyone else assigned that
pointer value.

I have seen no convincing argument as to why ++ is not supported in
Ruby.

  • Charlie

Yes, a++ and ++a could easily be rewritten by the parser into the
appropriate increment+set of a and the expression

Wouldn’t it be cool if ruby had macros?