Return values of modifiers

Ruby
1

This is logical, but no very useful :

irb(main):007:0> x = 10
=> 10
irb(main):008:0> x = x + 1 if false
=> nil
irb(main):009:0> x = x + 1 if true
=> 11

Idem, a while modifier always returns nil

irb(main):010:0> x += 1 while x < 15
=> nil

It would be nice to get the return value of the last instruction
before the test fails.

_md

2

2010/4/30 Michel D. [email protected]:

irb(main):010:0> x += 1 while x < 15
=> nil

It would be nice to get the return value of the last instruction
before the test fails.

Why? What use case is there that would require this or what code
would become (much) more elegant?

Note that the cost (in terms of memory) might be significant if the
expression before “while” returns a large volume of data on each
iteration. That’s probably irrelevant for most cases but it may cause
a nasty memory issues in rare cases because every last value needs to
be kept for the duration of the next loop execution.

Kind regards

robert

3

Robert K. wrote:

2010/4/30 Michel D. [email protected]:

irb(main):010:0> x += 1 while x < 15
=> nil

It would be nice to get the return value of the last instruction
before the test fails.

Why? What use case is there that would require this or what code
would become (much) more elegant?

I was just cleaning a library and found two methods ending with
something like

ind += 1 while <some_test>
ind
end

and tried to get rid of the last line, quite ugly and looking
unnecessary. You’re right : not much more elegant. But once you get
used to syntactic sugar, you’ll ask for some more … Remember Dijsktra
: “abuse of syntactic sugar gives cancer of the semi-colon”.
_md

4

David A. Black wrote:

In the case of the while in modifier position, it looks like it’s
equivalent to this:

while <some_test>
ind += 1
end

and it is. I prefer this to having it be equivalent to:

loop do x += 1; break x if <some_test>; end

David

Yes, that was my point.
_md

5

Hi –

On Fri, 30 Apr 2010, Michel D. wrote:

would become (much) more elegant?
used to syntactic sugar, you’ll ask for some more … Remember Dijsktra
: “abuse of syntactic sugar gives cancer of the semi-colon”.

I don’t think there’s any syntactic sugar here. I always think of
syntactic sugar as “It looks like it’s doing X, but it’s really [or
also] doing Y.” For example, x * y, where it looks like an infix
operator but is really sending a message to an object.

In the case of the while in modifier position, it looks like it’s
equivalent to this:

while <some_test>
ind += 1
end

and it is. I prefer this to having it be equivalent to:

loop do x += 1; break x if <some_test>; end

David

–
David A. Black, Senior Developer, Cyrus Innovation Inc.

THE Ruby training with Black/Brown/McAnally
COMPLEAT Coming to Chicago area, June 18-19, 2010!
RUBYIST http://www.compleatrubyist.com

6

Yes, that was my point.
_md
@David : Sorry, I misunderstood you, English is not my native tongue. I
read the opposite "I prefer it to be equivalent …
_md

7

Hi –

On Sat, 1 May 2010, Michel D. wrote:

loop do x += 1; break x if <some_test>; end

David

Yes, that was my point.

I thought you were saying you did want it to be equivalent to that
loop construct, rather than to the non-modifier while construct. Did I
misunderstand?

David

–
David A. Black, Senior Developer, Cyrus Innovation Inc.

THE Ruby training with Black/Brown/McAnally
COMPLEAT Coming to Chicago area, June 18-19, 2010!
RUBYIST http://www.compleatrubyist.com

8

Sure, it would be nice while/until loops return value of the last
iteration,
I agree.

9

Benoit D. wrote:

Sure, it would be nice while/until loops return value of the last
iteration,
I agree.

This boils down to the fact that ‘if false ; end’ returns nil, hence
means ‘do nil’ and not ‘do nothing’ (as a comment does).

Inserting a piece of code between ‘if false’ and ‘end’ is not equivalent
to commenting it out, and does not means ‘jump over it’. I feel it
should.

10

David A. Black wrote:

Hi –

On Sat, 1 May 2010, Michel D. wrote:

loop do x += 1; break x if <some_test>; end

David

Yes, that was my point.

I thought you were saying you did want it to be equivalent to that
loop construct, rather than to the non-modifier while construct. Did I
misunderstand?

No, you did not. This is was I suggested. But I misunderstood your first
comment, guessing you agreed with me. Sorry for the noise.
_md

11

On Sat, May 1, 2010 at 8:38 AM, Michel D. [email protected]
wrote:

Benoit D. wrote:

Inserting a piece of code between ‘if false’ and ‘end’ is not equivalent
to commenting it out, and does not means ‘jump over it’. I feel it
should.
The parser parses it, that is all, it is never executed I fail to see
what your desired behavior is?
Cheers
R.

12

Robert D. wrote:

On Sat, May 1, 2010 at 8:38 AM, Michel D. [email protected]
wrote:

Benoit D. wrote:

Inserting a piece of code between ‘if false’ and ‘end’ is not equivalent
to commenting it out, and does not means ‘jump over it’. I feel it
should.
The parser parses it, that is all, it is never executed I fail to see
what your desired behavior is?
Cheers
R.

13

Michel D. wrote:

I would like the return value of a block ‘if ; … ; end’ to be
the last return value before parsing the ‘if’. As if the block did not
exist, or was commented out.

I am not familiar with the ruby parser. What I suggest could be just to
pop the stack when exiting from the block.
_md

14

Michel D. wrote:

Robert D. wrote:

On Sat, May 1, 2010 at 8:38 AM, Michel D. [email protected]
wrote:

Benoit D. wrote:

Inserting a piece of code between ‘if false’ and ‘end’ is not equivalent
to commenting it out, and does not means ‘jump over it’. I feel it
should.
The parser parses it, that is all, it is never executed I fail to see
what your desired behavior is?
Cheers
R.

I would like the return value of à block ‘if ; … ; end’ to be
the last return value before parsing the ‘if’. As if the block did not
exist, or was commented out.

15

On Sat, May 1, 2010 at 9:30 AM, Michel D. [email protected]
wrote:

Michel D. wrote:

I would like the return value of a block ‘if ; … ; end’ to be
the last return value before parsing the ‘if’. As if the block did not
exist, or was commented out.

I am not familiar with the ruby parser. What I suggest could be just to
pop the stack when exiting from the block.
_md
It is impossible, how could the parser continue its work??

16

On 2010-05-01, Michel D. [email protected] wrote:

I would like the return value of à block ‘if ; … ; end’ to be
the last return value before parsing the ‘if’. As if the block did not
exist, or was commented out.

That would seem very surprising to me. In general, I do not expect the
return value of an expression to be the return value of some other
expression
which was in no way a part of it.

-s

17

What I meant is:

i = 0
=> 0
while i < 10
i += 1
33
end
=> nil # should be 33

This is already the case for class Foo…end, and if/unless

It should be for the loops too (without explicit break).

And when the condition is wrong and then the “block” is never used,
then nil should be returned.

What do you think?

18

Benoit D. wrote:

What I meant is:

i = 0
=> 0
while i < 10
i += 1
33
end
=> nil # should be 33

This is already the case for class Foo…end, and if/unless

It should be for the loops too (without explicit break).

And when the condition is wrong and then the “block” is never used,
then nil should be returned.

What do you think?

Benoit, if you have only
i = 0
while i < 10
i += 1
end
which is the same as
i = 0
while i < 10
i += 1
i
end
then it should return 10, as I wanted ?
_md

19

2010/5/1 Michel D. [email protected]:

i = 0
_md
Sure, that’s my point.

(But I think the ‘if/unless’ behavior is correct and “foo if false”
should return nil as it is)

20

Robert D. wrote:

It is impossible, how could the parser continue its work??

OK !