Having a block executed in the context of an instance


#1

I’m writing a property driven ui and would like to do things like:

button = Button.new do
caption = ‘Click Me’
end

I’ve read that 2.0 will have this feature, i.e. Class.new will execute
it’s
block argument in it’s own context. But how do I do it in 1.8?

Thanks much,
Mike


#2

On Wed, 8 Mar 2006, Mike A. wrote:

Mike
class Button
def initialize(*args, &block)
instance_eval &block
end
end

note that you’ll need to do

self.caption = ‘Click Me’

or else caption will be a local variable. for an elegant work around
see the
metakoan quiz or my traits library. with it a really nice syntax like
this is
possible

button = Button.new {
caption ‘Click Me’
colour ‘Orange’
size ‘42’
}

here’s a real example from something i’m coding now:

class Flo5 < NRT::OLSSubscription::Geotiffed
mode “production”

 roi 47,16,39,27

 satellites %w( F15 F16 )

 extensions %w( OIS )

 solarelevations -180, -12, 10.0

 hold 0

 username "flo"

 password "xxx"

 orbital_start_direction "descending"

 start_time "2004-12-08"
 end_time "2006-12-08"

 to_keep [ %r/.tif$/, %r/.hdr$/ ]

 tmpwatch "8 days"

end

hth.

-a


#3

removed_email_address@domain.invalid wrote:

self.caption = ‘Click Me’

Thanks, that works well. But as you note, prepending ‘self’ kind of
defeats
the purpose :wink: Is there a way to make Ruby not create local variables,
but use
‘local var’ or similar? Io (www.iolanguage.com) has this problem
early on
and was solved by using := for creating slots, and = for updating slots.
It
would be nice if it could be lenient when running in irb, and more
strict when
executing .rb files.

or else caption will be a local variable. for an elegant work around
see the
metakoan quiz or my traits library. with it a really nice syntax like
this is
possible

Metakoan quiz didn’t turn up anything. I looked at your traits library
and it
looks nice. I could use an operator instead of ‘=’ I suppose.

mode "production"


tmpwatch "8 days"

end

hth.

-a

Mike


#4

Mike A. wrote:

Thanks, that works well. But as you note, prepending ‘self’ kind of
defeats the purpose :wink: Is there a way to make Ruby not create local
variables, but use ‘local var’ or similar?

No, there isn’t. However, often using a hash as argument solves this
problem:

class Foo
def initialize(h={})
h.each {|var,val| send("#{var}=", val)}
end
end

Foo.new(
:caption => ‘Click me’,
:color => :orange,
:size => 42
)

With a bit more code you can even have default values.

Kind regards

robert

#5

button = Button.new do
caption = ‘Click Me’
end

Executing a block in another context isn’t very readable. Ara’s
example is not the same as your situation. His situation is an
example of a DSL, whereas your situation is just plain Ruby.
So, IMHO, it should be coded in just plain Ruby:

class Button
def initialize
yield(self)
end
end

button = Button.new do |b|
b.caption = ‘Click Me’
end

Now it’s obvious (by reading, not by knowing) what the block
does.

gegroet,
Erik V. - http://www.erikveen.dds.nl/


#6

In my opinion, this is unneccesary and still a bit confusing unless the
object has a limited lifetime (as in the case of IO resources) or will
change state when the block is ended. You could use:

button = Button.new
button.caption = ‘Click Me’

Blocks are good for DSLs or management of limited resources. It is a
bad idea to use the new method to provide a limited resource as this
will impair forward-compatability. A different method should be used
(and new possibly made private) to enable future change.

For button management I would use something along the lines of the DSL
outlined above but would use additional blocks in the same scope to
handle events.

button = Button.new do
caption “Click Me” # Public method, button.caption and
button.caption= useable from outside

on_click do
caption “You Clicked Me!” # Can update attributes in response to
events with very little extra work.
end
end


#7

“Mike A.” removed_email_address@domain.invalid wrote

Thanks, that works well. But as you note, prepending ‘self’ kind of
defeats the purpose :wink: Is there a way to make Ruby not create local
variables, but use ‘local var’ or similar? Io (www.iolanguage.com) has
this problem early on and was solved by using := for creating slots, and =
for updating slots.

I personally would like some such distinction. I don’t having to use
“self.att=” since
(a) one does not use self.method(…) in general
(b) self.method(…) has different privacy rules
© self.attr= is an exception to both (a) and (b)

Ara’s
attr(new_val)
works, but
(a) does not read like a setter, and
(b) is against the standard Ruby family of attr… methods

I wouldn’t hold my breath for := or equivalent :slight_smile:


#8

removed_email_address@domain.invalid wrote:

I have to say, I’ve been wondering for many years… is self.x = 1
really that bad? It just seems like such a minor thing to me. I
certainly wouldn’t want to see new punctuation introduced just to
avoid it.

+1


#9

Hi –

On Wed, 8 Mar 2006, itsme213 wrote:

“self.att=” since
I wouldn’t hold my breath for := or equivalent :slight_smile:
I have to say, I’ve been wondering for many years… is self.x = 1
really that bad? It just seems like such a minor thing to me. I
certainly wouldn’t want to see new punctuation introduced just to
avoid it.

David


David A. Black (removed_email_address@domain.invalid)
Ruby Power and Light, LLC (http://www.rubypowerandlight.com)

“Ruby for Rails” chapters now available
from Manning Early Access Program! http://www.manning.com/books/black


#10

removed_email_address@domain.invalid wrote:

removed_email_address@domain.invalid wrote:

I have to say, I’ve been wondering for many years… is self.x = 1
really that bad? It just seems like such a minor thing to me. I
certainly wouldn’t want to see new punctuation introduced just to
avoid it.

+1

It may be a minor syntax thing, but I think it creates unnecessary
ambiguity.
If there is a method called caption=(), I expect it to use that. I
don’t write
‘self’ anywhere else in Ruby, so it just seems odd. Plus is helps
reduce hard
to debug locals creation by misspellings.

class Button
attr :caption, true

def initialize(*args, &block)
instance_eval &block
end
end

button = Button.new do
var :x
caption = ‘Click Me’
end

There is already ‘attr’ for classes, why not ‘local’ or ‘var’ for
methods? One
issue is that you can’t assign and declare on the same line. It could
be done
with a hash but that might look funny?

Take this all with a grain of salt. I’m always looking for different
ways of
doing things. I.e. to find good ideas you have to find bad ideas :wink:

Mike


#11

On Mar 8, 2006, at 9:56 AM, removed_email_address@domain.invalid wrote:

I have to say, I’ve been wondering for many years… is self.x = 1
really that bad? It just seems like such a minor thing to me. I
certainly wouldn’t want to see new punctuation introduced just to
avoid it.

It isn’t that bad but it stands out since the rest of Ruby is
that good!

I think it is the old battle between ‘perfect’ and ‘good enough’.

Gary W.


#12

Hi –

On Thu, 9 Mar 2006, Mike A. wrote:

It may be a minor syntax thing, but I think it creates unnecessary ambiguity.
If there is a method called caption=(), I expect it to use that. I don’t
write ‘self’ anywhere else in Ruby, so it just seems odd. Plus is helps
reduce hard to debug locals creation by misspellings.

You can look at it the other way around, though: you always specify
a receiver explicitly in Ruby, except in the one case where the
receiver is self and the syntax is such as to be unambiguously a
method call.

caption = ‘Click Me’
end

There is already ‘attr’ for classes, why not ‘local’ or ‘var’ for methods?

I’m not seeing the connection between the two halves of that sentence
:slight_smile: I don’t think the existence of the attr_* methods implies that
Ruby should adopt a system of variable declaration. (At least, I
don’t see how one follows from the other.) But I’m not sure I’m
understanding you correctly.

David


David A. Black (removed_email_address@domain.invalid)
Ruby Power and Light, LLC (http://www.rubypowerandlight.com)

“Ruby for Rails” chapters now available
from Manning Early Access Program! http://www.manning.com/books/black


#13

On Mar 8, 2006, at 2:09 PM, Dominik B. wrote:

certainly wouldn’t want to see new punctuation introduced just to
It’s no “new” punctuation and I can’t see any ambiguities with
current ruby syntax (correct me if I’m wrong).

Just an idea.

Dominik

Hmmmmmm, It’s shorter than self.attr, but on the other hand it’s
uglier IMO. One question I would have to wonder is:

self.method( ) # definitely not private
method( ) # definitely private
self.x = y # could be public or private
.x # probably(?) not private ???
.x = y # ???

Oh wait I take the whole thing back, naked method( ) could be public
too. I guess throwing .x in there doesn’t make the visibility any
more confusing


#14

On 3/8/06, Mike A. removed_email_address@domain.invalid wrote:

def initialize(*args, &block)
instance_eval &block
end
end

button = Button.new do
var :x
caption = ‘Click Me’
end

If you don’t mind dropping the equals sign it’s a pretty easy fix. At
that
point you could even do some method_missing magic and have it check for
existing attributes and use those.

There is already ‘attr’ for classes, why not ‘local’ or ‘var’ for

methods? One
issue is that you can’t assign and declare on the same line. It could be
done
with a hash but that might look funny?

Take this all with a grain of salt. I’m always looking for different ways
of
doing things. I.e. to find good ideas you have to find bad ideas :wink:

And as david said, really the lack of a receiver is the exception not
the
rule.

Mike


#15

On Wed, 08 Mar 2006 15:56:53 +0100, removed_email_address@domain.invalid wrote:

Hi –

On Wed, 8 Mar 2006, itsme213 wrote:

I wouldn’t hold my breath for := or equivalent :slight_smile:

I have to say, I’ve been wondering for many years… is self.x = 1
really that bad? It just seems like such a minor thing to me. I
certainly wouldn’t want to see new punctuation introduced just to
avoid it.

How about allowing

.attr = val

as a shortcut for

self.attr = val

It’s no “new” punctuation and I can’t see any ambiguities with current
ruby syntax (correct me if I’m wrong).

Just an idea.

Dominik


#16

removed_email_address@domain.invalid wrote

You can look at it the other way around, though: you always specify
a receiver explicitly in Ruby, except in the one case where the
receiver is self and the syntax is such as to be unambiguously a
method call.

Not quite. I believe
self.foo
does completely different privacy checks than just
foo

Specifically,
self.foo
on a private foo will fail, while
self.foo=
on private foo= will succeed.


#17

Hi –

On Thu, 9 Mar 2006, itsme213 wrote:

does completely different privacy checks than just
foo

Specifically,
self.foo
on a private foo will fail, while
self.foo=
on private foo= will succeed.

I don’t think that’s an exception to what I said, though. A call to a
private (non-=) method falls into the category I’m describing: the
receiver is self, and the syntax is such as to be unambiguously a
method call.

David


David A. Black (removed_email_address@domain.invalid)
Ruby Power and Light, LLC (http://www.rubypowerandlight.com)

“Ruby for Rails” chapters now available
from Manning Early Access Program! http://www.manning.com/books/black


#18

Hi –

On Thu, 9 Mar 2006, Logan C. wrote:

Hmmmmmm, It’s shorter than self.attr, but on the other hand it’s uglier IMO.
One question I would have to wonder is:

self.method( ) # definitely not private
method( ) # definitely private
self.x = y # could be public or private
.x # probably(?) not private ???
.x = y # ???

Oh wait I take the whole thing back, naked method( ) could be public too. I
guess throwing .x in there doesn’t make the visibility any more confusing

I wonder how the parser would handle:

obj .x

i.e., whether it would be:

obj.x

or

obj(.x)

David


David A. Black (removed_email_address@domain.invalid)
Ruby Power and Light, LLC (http://www.rubypowerandlight.com)

“Ruby for Rails” chapters now available
from Manning Early Access Program! http://www.manning.com/books/black


#19

On Mar 8, 2006, at 9:58 PM, removed_email_address@domain.invalid wrote:

from Manning Early Access Program! http://www.manning.com/books/black

I would imagine obj.x since that is the current behavior


#20

Dominik B. wrote:

meth .attr=42

I would say that would also be meth.attr=42 and not meth(.attr=42).

The spacing, though, makes it look like the latter. The spacing really
makes it look like 42 goes with .attr, not with meth.

Of course, then there is:

meth . attr = 42

and other fun variations :frowning: